WASHINGTON (AP) _ Daniel Patrick Moynihan, who rose from New York's Hell's Kitchen to become a Harvard professor and one of Congress' authorities on welfare and Social Security, will retire from the Senate when his term expires in two years, Senate aides said today.

The 71-year-old Democrat, with his trademark white shock of hair and slow professorial manner of speech, is serving his fourth six-year term and would have been expected to be re-elected easily in 2000.

The aides who revealed Moynihan's decision spoke on condition of anonymity.

Word of Moynihan's retirement came just three days after his long-time New York colleague, Sen. Alfonse D'Amato, R-N.Y, was defeated in his bid for a fourth term.

For years, the two men complemented each other with Moynihan focusing on foreign affairs, social policy and other broad issues and D'Amato concentrating on bringing home projects to his state.

Moynihan is the first senator facing re-election in 2000 to announce his retirement. With Republicans failing to expand their 55-45 Senate majority in Tuesday's voting, Democrats have been hoping to recapture control of the chamber that year, when 19 Republican and 14 Democratic seats will be at stake.