Salt, a chemical that combines the elements sodium and chlorine, has occupied a foremost position in human society since prehistoric times. The Bible speaks approvingly of ''the salt of the earth.'' A good Greek slave was said to be ''worth his weight in salt.''

Salt has been bartered, used for pay, and fought over. A tax on it was sure to bring violent protest. It once was worth its weight in gold literally, traded ounce for ounce.

Salt also has been a traditional symbol of social status. The most important guests at the dinner table were seated ''above the salt.''

In recent years, however, research has linked excessive consumption of salt to hypertension (high blood pressure) and its potentially fatal consequences, heart and kidney disease and stroke.

Hypertension, one of the nation`s most widespread diseases, afflicts an estimated 60 million Americans, usually producing no symptoms until one day the signs of permanent organ damage appear as a chronic illness or death.

The main precipitant of hypertension, much evidence indicates, is the mineral sodium, which is 40 percent of the salt molecule by weight. Between 15 and 20 percent of Americans are genetically prone to developing high blood pressure if their diet is rich in sodium, as the typical American diet is.

Hypertension is practically non-existent among people in cultures in New Guinea, the Amazon Basin, the Kalahari Desert and elsewhere where little or no salt is added to foods. In such cultures blood pressure does not rise

''naturally'' with age; if anything, it drops.

Some say it is lack of stress, not lack of salt, that keeps the lid on blood pressure in these cultures. But in a few pre-industrial societies such as the Gashgai nomads of southern Iran, where a lot of salt is consumed, high blood pressure is common despite the lack of societal tensions.

On the other hand, in societies where salt and sodium are consumed to great excess, such as in Japan, hypertension is the leading cause of death and disability. Further, the ravages of hypertension are greatest in northern Japan where salt consumption is highest.

When 1,346 Americans were grouped according to salt intake more than 10 percent of those who consumed a lot of salt were found to have high blood pressure, 7 percent of those who consumed an average amount of salt had the condition, but less than 1 percent of those who consumed a small amount of salt had hypertension.

A five-year study at the Mayo Clinic showed that sodium restriction alone could lower blood pressure to normal in persons with mild hypertension.

Studies of laboratory rats suggest that excess salt early in life can set the stage for later development of high blood pressure. If rats that are genetically predisposed to hypertension never are given salt, their blood pressure remains normal throughout life.

But those that get salt early in life develop hypertension even if the salt is later eliminated from the diet. Unfortunately, it is not possible to determine in advance which people are susceptible to the pressure-raising effects of salt.

Sodium is a vital constituent of the human body. Our tissues swim in a salty sea-a vestige perhaps of our aquatic evolution. The more salt in the sea, the more water is needed to dilute it to maintain the proper

concentration of sodium.

WORK OF KIDNEYS

The body`s machinery for keeping a normal level of sodium in its fluids is the kidneys. When the body has too much sodium the kidneys dump it out into the urine and excrete it. When the body needs sodium the kidneys reabsorb it from urine and pump it back into the blood.

Unfortunately, in a significant percentage of people, perhaps as a result of having to dump excess sodium for years, this machinery fails to operate properly and the kidneys don`t get rid of enough sodium. The retained sodium holds water and the volume of blood rises. The blood vessels become waterlogged and more sensitive to nerve stimulation that causes them to contract.

Because more blood now has to pass through the same ever-narrower channels, the blood pressure increases. The heart rate also increases because the heart has more blood to pump around the body. This in turn sets up a vicious cycle in which the blood vessels contract to reduce the blood flow. The pressure then rises even further until you have hypertension.

While this is a vast oversimplification of a mechanism that is not understood completely, it gives you some idea of how excess consumption of sodium can cause high blood pressure in susceptible persons.

Stress adds further to the problem by stimulating the release of an adrenal-gland hormone, aldosterone, which signals the kidneys to hold on to sodium and water. But even in the absence of stress, too much sodium can do serious damage.

In addition to increasing the volume of blood, excess sodium increases the amount of water in and around body tissues. This results in swelling, or edema. When the swelling occurs around the heart the result can be congestive heart failure-the heart can`t pump properly.

Swelling in the legs may interfere with the return of blood to the heart and result in difficulty walking and a tendency to form clots in the veins. Swelling of tissues in the pelvic region can increase the discomfort associated with menstruation. And swelling of tissues surrounding the brain may cause emotional disturbances such as depression and irritability.

The body`s tendency to retain sodium just before the start of the menstrual period is the primary cause of premenstrual weight gain and the distressing symptoms of premenstrual tension-bloating, headache and irritability.

Many women find that they crave salt just before their period. If they satisfy that craving it makes matters worse because their tissues swell even more than they might otherwise. For women bothered by premenstrual problems and menstrual cramps, doctors recommend a low-salt diet for a week to 10 days before the period is due.

It long has been known that pregnant women frequently have salt cravings

(witness the infamous desire for pickles in the middle of the night). And it recently has been shown in laboratory rats that oral contraceptives, which mimic the hormonal state of pregnancy, increase the animals` appetite for salt.

This may explain the weight gain commonly associated with taking the birth control pill. Without even realizing it, women on the pill may consume more salt and this, in turn, holds more water in their bodies. The extra pounds are not fat, but water weight.

Too much sodium in body fluids also causes water and potassium to be drawn out of the body`s cells. Undoubtedly you`ve noticed what happens when you sprinkle salt on cucumbers or lettuce. They wilt because the water in their cells is drawn out by the salt.