Lawrence A. Kudlow, chief economist at the Office of Management and Budget and a close adviser to Director David Stockman, plans to leave his post sometime in the middle or latter part of next month.

Kudlow, who came from the Wall Street investment firm Bear, Stearns and Co. and was a key architect of President Reagan's economic program, said yesterday, "I'm going to go into business with some friends . . . I will try my luck as an entrepreneur." He said he had always planned to leave the administration after about two years, and commented "I can only say it's been a fascinating experience . . . I'm very pleased about how things have worked out."

In the early days of the administration, Kudlow was very optimistic about the prospects for the economy under President Reagan's budget policies. However, he later argued that some economic slowdown was an inevitable part of the fight against inflation. He also has been critical of the large budget deficits to which the president's policies have led.

More recently, Kudlow has reportedly been a strong critic of the rapid growth in the money supply that has accompanied the decline in interest rates since last summer. He declined to comment on this yesterday, saying, "when I get out in the private sector I'll have plenty of time" to write about the economy. He said he believed "policies are on the right track."

No successor has yet been chosen, Kudlow said.