An 18-year-old Kingston, Jamaica, man convicted of second-degree murder in the drug-related shooting of a District resident was sentenced to 50 years in prison yesterday by a Montgomery County Circuit Court judge.

Donovan George Brown, who was 17 at the time of the slaying, was sentenced by Judge L. Leonard Ruben to 30 years' imprisonment on the murder charge, and a consecutive 20-year term on the related charge of using a handgun in the commission of a crime. He will be eligible for parole in 12 years.

"Obviously Mr. Brown didn't show the victim in this case any leniency," Judge Ruben commented.

Brown, an alleged marijuana dealer, was convicted by a jury Jan. 30 of shooting 30-year-old George Fratta Lewis in the head on Dec. 3, 1984.

Prosecutors relied on the testimony of an admitted drug courier, Earl (Jackie) Wallace, who told the jury that he saw the murder at the Silver Spring apartment he shared with two other dealers. Lewis' body was found the next day, rolled in a carpet, by construction workers at a Burtonsville development.

Prosecutors claimed that Brown killed Lewis in revenge because he suspected Lewis of stealing a cache of drugs. Assistant State's Attorney Ann S. Harrington said yesterday that Brown was "self-anointed to determine whether that person had committed crimes against him and his business interests . . . . "

But defense attorneys urged Ruben to consider Brown's youth, his neglected childhood and the "bad influence" of his friends.

"This is a country boy from Jamaica," said Public Defender Eugene Wolfe, "caught up in the fast life . . . . "

Brown, who did not testify at his trial, said, "I ain't guilty. I never kill anybody in my life before." But, he muttered, "It's up to you, anyway."