NEWARK, N.J. (AP) _ At 50, Barbara Schein has started her first full-time job.

She's a rookie police officer patrolling the northern New Jersey borough of North Haledon in a cruiser.

Her first two weeks on the job were fairly quiet, she said. The hard part was becoming an officer.

Mrs. Schein had to complete 4 1/2 months of grueling physical training with men half her age before she graduated from the Bergen County Police Academy.

''I'd never go through it again,'' she said recently. ''If I wasn't so stubborn, maybe I wouldn't have made it. But I really wanted it. It was something I had to prove to myself and my chief.

''It's something I wouldn't dare think of not completing. They would have had to carry me out in a box.''

Mrs. Schein said she has wanted to be a police officer all her life but didn't pursue it as a career when she was younger because ''they didn't take many women.'' She spent most of her married years as a housewife rearing her two daughters and selling insurance part-time.

Later, she inquired about a police career but found out that she was over the state age limit of 35. She settled for being a ''special officer'' for 12 years, directing traffic and helping maintain order at local basketball games.

Several years ago, the age limit was deemed discriminatory and dropped. When Mrs. Schein learned of an opening with the North Haledon force, she took and passed the written test in June.

She then began working out at a gym to increase her upper body strength and two months later entered the police academy.

Mrs. Schein, who is 5 feet tall, said she has always exercised daily, but being in ''pretty good shape'' did not make the physical training any easier.

After running up to five miles a day, performing an hour of calisthenics, attending classes and then running an obstacle course, Mrs. Schein said she went home and yelled at her husband, Edward.

''He never thought it would end,'' she said, laughing heartily.

However, her husband supported her efforts, she said.

''Basically, I do whatever my heart desires. I've been married for 32 years and when you get married, it's 50-50. No man will tell me I can't do something,'' she said.

Her 6 1/2 -year-old grandson ''thinks it's great'' to have a police officer for a grandmother, she said. She also has an infant granddaughter.

Mrs. Schein said other officers also have backed her.

''There were 92 who graduated with me and there were only five in the whole class that I didn't get along with,'' she said. ''There are some people who just don't think women should be police officers.''

Mrs. Schein's chief, Edward Dombroski, said he believes the majority of his 15-officer department has accepted her.

''They're waiting to see if she can handle her own,'' he said. ''I believe she can.''

Despite some others' skepticism, he said, he always believed Mrs. Schein would make it through training.

''She's a very strong-willed person,'' he said. ''When she decides to do something, she sets out for that and she accomplishes it.''

Mrs. Schein's accomplishment is rare in New Jersey. According to the state police academy, he said, a few men have become rookie officers at age 50 or older, but no women until Mrs. Schein.

Of New Jersey's 28,958 municipal, county, state and university police officers, 1,259 are women, according to 1985 figures compiled by the state Police Training Commission. There is no mandatory retirement age for municipal officers.

Mrs. Schein said she has no particular goals as an officer, ''just to do a good job and have 10 to 15 good years.''