Calling him a "Dr. Jekyll and Mr. Hyde," a Fairfax County judge sentenced Brian S. Kunkle to 45 years in prison yesterday for sexually assaulting five women while they were working alone at night in office buildings.

Circuit Court Judge Barnard F. Jennings said that despite an apology from Kunkle, a plea for mercy from Kunkle's mother and letters from people saying he was a "capable, responsible person," there was no excuse for committing the offenses he was convicted of: aggravated sexual battery, two counts of attempted abduction with intent to defile, attempted aggravated sexual battery and rape.

Before Kunkle, 28, was sentenced, one of the women he assaulted told the judge of experiencing "utter humiliation" when she was attacked as she left a Reston office building.

"I really need to tell you about the impact this had on me," said the 30-year-old woman, dissolving into tears. She said Kunkle, who was masked, stripped her and made her "do stupid things like turn around" while he looked at her and touched her. "I thought he was going to kill me. I thought I was going to die."

Assistant Commonwealth's Attorney Lorraine Nordlund, who prosecuted the case, said she was pleased with the sentence, which she called appropriate.

Kunkle pleaded guilty last January. At that time, all the other women he had accosted testified that they had been working alone at night between March 1984 and October 1985 in office buildings in the Reston or Tysons Corner areas when they were attacked.

Nordlund said the 30-year-old woman who spoke at yesterday's court proceeding had boarded up all the windows in her three-story house out of fear.

Police said Kunkle was arrested after accosting a police decoy, JoAnne G. Studer, who had been placed in an office building where an attack occurred. Judge Jennings praised Studer yesterday for "subjecting herself to this treatment" and said that he'd never seen a case more thoroughly prepared.

Nordlund said Kunkle will be eligible for parole in 10 to 12 years.