NEW YORK (AP) _ A writer whose column was dropped by Forbes magazine after his research was challenged is now the subject of a state probe, according to a report published today.

The state Office of Professional Discipline has begun a probe of Srully Blotnick and the possibility that he engaged in the unauthorized practice of psychology, the New York Daily News reported.

Blotnick, 46, is the author of five successful books and hundreds of articles which are based, he has said, on a 29-year-survey of between 5,000 and 10,000 Americans.

His books include ''The Corporate Steeplechase,'' about career patterns of 5,000 Americans, and ''Otherwise Engaged,'' a study of career women in the United States. He also had a column in Forbes magazine since 1979 until it was discontinued Monday.

Research experts have challenged Blotnick's data, saying that tracking so many people would be expensive and require a large staff.

Blotnick previously has said that he and his co-workers, who he did not identify, had interviewed the group twice a year since 1958.

Blotnick would have been 17 and a freshman at the University of Miami when the study began. When asked how he went to school full time, became the director of research at three Wall Street brokerage houses and kept up the study at the same time, Blotnick said: ''With help.''

But, Blotnick now says that he used a ''nodal'' research method, in which a single individual would be interviewed about the entire family, the News reported. Under that method, information about six people could be obtained from one person.

Blotnick dropped the ''Dr.'' in his magazine byline several years ago, according to Forbes editor James Michaels, although he continues to use it on his books. He had called himself a ''practicing psychologist,'' ''research psychologist'' and ''business psychologist'' in the column.

He received a Ph.D. in psychology in 1978 from Pacific Western University, an unaccredited correspondence school in Los Angeles, the News said.