"I was completely astonished by the beauty of nature," he said, laying a slightly gnarled finger alongside his nose, his longish white hair swept back from his temples and the crown of his head. He said any natural scientist who was not a mystic was not a real natural scientist. "Outside is pure energy and colorless substance," he said. "All of the rest happens through the mechanism of our senses. Our eyes see just a small fraction of the light in the world. It is a trick to make a colored world, which does not exist outside of human beings."

He became particularly fascinated by the mechanisms through which plants turn sunlight into the building blocks for our own bodies. "Everything comes from the sun via the plant kingdom," he said.

MR. HOFMANN studied chemistry and took a job with the Swiss pharmaceutical company Sandoz Laboratories, because it had started a program to identify and synthesize the active compounds of medically important plants. He soon began work on the poisonous ergot fungus that grows in grains of rye. Midwives had used it for centuries to precipitate childbirths, but chemists had never succeeded in isolating the chemical that produced the pharmacological effect. Finally, chemists in the United States identified the active component as lysergic acid, and Mr. Hofmann began combining other molecules with the unstable chemical in search of pharmacologically useful compounds.

His work on ergot produced several important drugs, including a compound still in use to prevent hemorrhaging after childbirth. But it was the 25th compound that he synthesized, lysergic acid diethylamide, that was to have the greatest impact. When he first created it in 1938, the drug yielded no significant pharmacological results. But when his work on ergot was completed, he decided to go back to LSD-25, hoping that improved tests could detect the stimulating effect on the body's circulatory system that he had expected from it. It was as he was synthesizing the drug on a Friday afternoon in April 1943 that he first experienced the altered state of consciousness for which it became famous. "Immediately, I recognized it as the same experience I had had as a child," he said. "I didn't know what caused it, but I knew that it was important."

When he returned to his lab the next Monday, he tried to identify the source of his experience, believing first that it had come from the fumes of a chloroform-like solvent he had been using. Inhaling the fumes produced no effect, though, and he realized he must have somehow ingested a trace of LSD. "LSD spoke to me," Mr. Hofmann said with an amused, animated smile. "He came to me and said, 'You must find me.' He told me, 'Don't give me to the pharmacologist, he won't find anything.' "