The president of Alex. Brown Inc., the region's oldest securities firm and one of its largest, yesterday announced that he will resign his position but will stay at the firm in a "non-management" job.

Donald B. Hebb Jr., who has worked at the Baltimore company for 20 years, including seven as president, told the board of directors of his decision Monday night. He cited the need for new management that can bring "fresh insights into moving the firm forward," according to an Alex. Brown spokesman.

Alex. Brown expanded rapidly under Hebb's stewardship, growing from a firm with $132 million in revenue and 1,000 employees in 1984 to one with $300 million in revenue and 1,700 employees at the end of 1989. But the growth hasn't been painless, especially with the economic slump that has hit the securities business.

Alex. Brown recently announced it expected to lay off at least 30 investment bankers, researchers and traders, or about 2 percent of its work force. For the three months ended Sept. 30, it reported a loss of $3.1 million, compared with a profit of $6.3 million for the same period in 1989. But since going public in 1986, the firm has never reported an annual loss.

Hebb, 48, will continue as president for several months until a replacement is found, a company spokeswoman said. Hebb will then take control of many of Alex. Brown's investment banking operations, including its mergers and acquisitions and merchant banking divisions, she said.