Mark Landstreet, a 26-year-old former file clerk who pleaded guilty last month in four sexual assaults, was sentenced to 43 years in prison yesterday by Fairfax Circuit Judge Thomas J. Middleton, who called him "a human volcano of explosive sexual urges."

Middleton, who could have imposed up to four life terms plus 135 years for the dozen felonies -- including sodomy, abduction and assault with intent to defile -- to which Landstreet pleaded guilty, listened for more than an hour to testimony indicating Landstreet suffers from a progressive psychiatric disorder and cannot control his violent sexual impulses.

Then, just before the judge imposed sentence, Landstreet, his voice choking, said haltingly:

"I pray by the grace of God that the victims can overcome what trauma I've caused them. If there is any way I can help remove their pain, I would welcome it . . . I despise the acts I've done."

Middleton responded: "I am aware you confessed to these crimes, I know you are remorseful and I know you have a sense of personal grief, but I have to view these offenses in the broad spectrum of things.

"I believe you are a very dangerous, very violent man," Middleton continued. "You are a human volcano of explosive sexual urges."

Mark E. Sharp, Landstreet's attorney, and assistant prosecutor Lorraine Nordlund said they were satisfied with the sentence.

Middleton actually imposed 118 years in prison, but 75 of those years are to run concurrently with the remaining 43. Landstreet will be eligible for parole in about seven years.

He was charged in four assaults on females ranging in age from 11 to 50, according to Nordlund. One of those assaults occurred in 1981 and the other three last summer and fall.

Landstreet was arrested last Oct. 28 in the process of assaulting a woman on the sidewalk outside her apartment building. Nordlund said Landstreet then told police he was involved in the four incidents in which he was subsequently charged plus "10 to 15 others."

Sharp asked Middleton to place Landstreet in a program in which some sex offenders are given injections of Depo-Provera, a controversial drug designed to change hormonal balance and lower sex drive. "I think the court knows these problems are amenable to treatment," Sharp said. "I urge you not to just throw him away, not to just throw him on the ash-heap of humanity."

Middleton said he will write letters to prison officials recommending Landstreet be placed in Virginia's sexual offenders program, and also recommending that he be made eligible for Depo-Provera treatment.