Lawrence Iason, head of the Securities and Exchange Commission's New York Regional Office - the Wall Street watchdog agency's largest and busiest - is resigning to return to private law practice effective March 15. Iason, who took over the New York office Jan. 25, 1988, said today he told SEC Chairman Richard Breeden last week he planned to leave government service to return to his old law firm. ``The time has come. I'm ready to go back to private practice,'' Iason said in a telephone interview from his office in Manhattan. He will be a partner at the New York firm of Morvillo, Abramowitz and Grant. Otto Obermaier, the U.S. attorney in Manhattan, is a former partner in the firm. As regional administrator of the New York office, Iason presided over several key investigations including the Business Week insider trading cases stemming from leaks of upcoming market sensitive stories and an administrative action against the brokerage firm Donaldson, Lufkin & Jenrette Securities Corp. that led to a $5.6 million settlement for alleged misuse of customer accounts. Perhaps more significantly, Iason took over an office in disarray after one regional administrator left suddenly in 1987 and officials from SEC headquarters in Washington ran the office on an acting basis. One of them, William McLucas, now heads the SEC's Enforcement Division. ``I'm sorry to see him go. I think what he did with that office is remarkable,'' McLucas said of Iason. Iason, 45, is a 1967 Yale graduate who got his law degree in 1971 at New York University. He clerked for a federal judge, served in the Watergate special prosecutor's office and the U.S. attorney's office in New York before joining his former law firm for the first time in 1979. As head of the regional office he oversaw a staff of 250 that includes, secretaries, investigators and lawyers. A native of Hewlett, N.Y., Iason now lives with his wife and three children in Mamaroneck, N.Y.