A Wall Street stalwart and a philanthropist for whom Carnegie Hall’s newest concert venue is named, Arthur Zankel, fell to his death yesterday from his ninth-floor apartment on the Upper East Side. Police called it an apparent suicide.

With his wife present at his home at 920 Fifth Ave., Zankel, 73, plunged from a window at the rear of the building at 11 a.m. and landed in the central courtyard, police said. Police said Zankel had a pulse when he was taken to New York Hospital but died there.

Friends of Zankel’s reacted to his death with shock and described him as a man with a fiendish sense of humor who was at the apex of his career and who beamed about his marriage to his third wife, Judy Zankel, an illustrator, and their new apartment overlooking Central Park.

Five years ago, after leaving First Manhattan and entering semi-retirement, he formed a real estate investment trust called High Rise Capital Management that profited greatly during the real estate boom.

Friends, speaking on background, said Zankel suffered from various ailments, including kidney stones and a rare blood disorder, and was in pain. He was said to have experienced bouts of depression associated with his medical difficulties.

News of his death quickly spread across Wall Street and reverberated around the myriad educational and cultural institutions that Zankel supported.

“Arthur Zankel will be remembered as a kind, caring, humorous, brilliant, and wise person,” a prominent attorney, Kenneth Bialkin, said.

Mr. Bialkin, who served on the board of directors of Citigroup along with Zankel, described his close friend as a “classical investor” who was “one of the most respected figures in the financial world.”

Zankel was known as a behind-the-scenes master of the market. He made a fortune as an early investor in Berkshire Hathaway, the Warren Buffett company, and served for many years as co-managing partner of an investment banking firm, First Manhattan Company.

A longtime friend and adviser to Citigroup’s chairman, Sanford Weill, Zankel was an important player in the historic merger of Citibank and Travelers Group insurance company in 1998.

Mr. Weill, who first met Zankel in 1957, told The New York Sun yesterday in a telephone interview Zankel “really understood the numbers” and was able to “dissect the details of a transaction and could catch things that didn’t make a heck of a lot of sense.”

“From my point of view, he was probably more helpful to me in what I was able to accomplish than maybe anybody,” Mr. Weill said.

For most of his career, Zankel remained a relatively unknown figure outside the investment world. Many of his donations, including those that established charter schools in Brooklyn, were made anonymously, friends said.

It was not until the construction of Judy and Arthur Zankel Hall at Carnegie Hall that Zankel gained name recognition. He donated $10 million toward the construction of the underground concert auditorium, which opened in September 2003.

A relatively small space with a seating capacity of 644 people, Zankel Hall was designed to be a more intimate showcase for lesser-known musicians. Mr. Weill said he solicited the donation and persuaded Zankel to allow Carnegie to attach his name to the concert hall.

“I suggested it would be great to name this hall for him, and in return, he’d have to do something significant,” Mr. Weill said.

“He never needed publicity,” Mr. Weill said. “The last eight years of his life, and his marriage to Judy, was the happiest he had. That was part of the excitement.”

At the time of his death, Zankel was vice chairman of the board of Teachers College, one of a number of educational and cultural institutions whose investment committees he oversaw.

He also served as trustee at Skidmore College, alma mater of two of his children, and of the Jerusalem Foundation and UJA-Federation. He served on the board of White Mountains Insurance Group.