The real-estate market is Mr. Trump's thermometer for gauing just how 'hot' a city is. 'New York is, right now, perhaps the hottest city ever,' he says. Recalling recent years when Paris, London, Los Angeles and Chicago had been hot, 'at some point, real estate here will have to go down, but that point is not in sight. One element that makes the market stronger here than in other U.S. cities is the Europeans, South Americans and others.'

Arriving in his office on the 26th floor of Trump Tower, Donald Trump is handed a sprial notebook by his secretary, Norma I. Foerderer, that lists about 50 telephone calsshe has received this morning that she deems worth mentioning. 'It's carzy,' he remarks. 'People are coming to me now because I have credibility.' He says he senses it is ephemeral. He is seizing the moment.

Mrs. Foerderer and a few others guard the ramparts, beating back dozens, sometime hundreds, of callers each day who would like to throw in with Mr. Trump n a variety of deals. Visitors are treaed to a slide show on Trump Tower while they wait - with superlatives by The Trump Organization and vocal accompaniment by Frank Sinatra. In their efforts to get through to Mr. Trump, some of the visitors tell Mrs. Foerderer they are old buddies of his, others bring candy and flowers. They want to propose marriage to Mr. Trump or to put a tank of dolphins in the lobby or have him back a Hollywood film or do a television series about rich people living in Trump Tower or sell him some oil wells in Oklahoma or some land in Ankara or ask if he would be interested in a plan to bulldoze Ellis Island to build a nice golf course and clubhouse out there. Some people try to make it simple for him and just ask for cash. The day before he has sent $3,000 to an unfortunate family he has red about in the newspaper, something he does frequently, according to Mrs. Foerderer.

For a billion-dollar corporation, there aren't too mny people around. Mr. Trump runs The Trump Organization, which includes several companies that buy, sell and develop land, own land and buildings, and a company, now inactive, that bought and sold gold, which, Mr. Trump confirmed, reaped him a $32 million profit. Mr. Trump owns all of these. He is a 50-50 partner in companies that own the Gran Hyatt hotel, Trump Tower and Harrah's at Trump Plaza. He owns 90 percent of the Trump Plaza cooperative building partnership. The Trump family owns 25,000 apartment units primarily in Brooklyn, Queens and Staten Island - the empire that Fred C. Trump, Donald Trump's father, built. The elder Mr. Trump looks after these apartments from an office at the rear of an apartment building at 600 Avenue Z in Brooklyn.

Fred Trump's empire, which he built from scratch, had an estimated value of $40 million when Donal joined the business 16 years ago. Donald's brother, Robert, is an executive vice president of the organization. (An older brother, Fred Jr., died several years ago.) His two sisters are Maryanne Trump Barry, a Federal Distict Court judge in Trenton, and Elizabeth J. Trump, a secretary at the Chase Manhattan Bank. They were raised in a 23-room house in Jamaica Estates. The family is of Swedish descent.

Donald Trump makes or approves practically all decisions. Although there is a board room, there is no board. At the moment, he is telling a doorman on the other end of the telephone not to put that tacky runner down on the eautiful marble floor when it rains. He does not seem to write anything down, keeping volumes of company files as mental notes.

Mr. Trump's wife, Ivana, is also an executive vice president of the company and has an office next door to her husband's. She is a former fashion model - 'a top model,' in Mr. Trump's words - who was married to Donald Trump seven years ago by the family's minister, the Rev. Norman Vincent Peale.