A top trader at Drexel Burnham Lambert Inc. who turned bullish on Black Monday with a reporter at his side has resigned, Drexel confirmed yesterday.

At 12:35 p.m. on Oct. 19, with the Dow Jones industrial average down 177 points and with a Washington Post reporter present to chronicle his day, a buoyantly optimistic Mark Mehl, director of institutional stock trading at Drexel, bet that the market had bottomed out.

"Let's put our money where our mouth is and find some stock to buy," he told his traders in Drexel's open stock trading room.

About an hour earlier he had shouted to the traders, "All you little weasels! I want your orders!" Then he turned to a colleague and said: "Did you cancel that vacation? We've got a lot of work to do this week. We're going to make a few million dollars."

But in the next few hours the Dow continued to fall, down 350 points, then 400. "I can't imagine it going any lower than that," he said.

The plunge accelerated until the close of the market, when the Dow landed 508 points below its start. "I've been a little humbled," the 35-year-old trader said.

In a reflective moment afterward, Mehl said: "I was a superstar for four hours. Then I was a {expletive}."

Drexel Chairman Robert E. Linton, confirming a Wall Street Journal report yesterday that Mehl had resigned, said, "He didn't resign because of the {Post} article."

But Linton added, "It was unfortunate we had a reporter in the room at all at a time like this."

A spokesman for Drexel said that Mehl, who joined the firm in 1986 after working for Salomon Brothers, resigned "as a result of policy differences related to equity trading strategy."

Mehl could not be reached for comment.