Monica Lewis, the 5-year-old Silver Spring girl whose doctors feared would die soon without a new liver, went into surgery for a transplant operation last night at Children's Hospital in Pittsburgh.

The operation, which was to last eight hours, began at 9 p.m., according to a hospital spokesman, who said there would be no further announcements until 9 this morning. The girl was flown to the hospital's famed transplant center on Monday after her condition worsened last weekend.

Earlier yesterday, Dr. Basil J. Zitelli, a pediatrician at Children's, said: "All systems are go. We have matched Monica with a potential donor and have permission from the the donor's family to harvest the organ."

Monica's operation will make her the 59th liver recipient at Pittsburgh Children's Hospital since 1981.

Hospital officials yesterday would not release details about the donor's identity or cause of death.

Monica suffers from a hereditary disorder called Alpha-1-Antitrypsin, which causes severe scarring of the liver tissue, rendering the organ virtually useless in cleansing the blood of poisons. One common side effect is dangerous internal bleeding. Doctors said Monica's bleeding has steadily worsened since December and prompted her flight to Pittsburgh this week with her parents, Mike and Pat Lewis.

"She had stabilized very well for a couple of days, but had more gastrointestinal bleeding Thursday night," Zitelli said. Last night's operation was to be led by Dr. Byers Shaw, an associate of Dr. Thomas E. Starzl, the country's leading liver-transplant surgeon.

A family friend who spoke with Monica's father yesterday said he sounded relieved about the operation. "He did not sound in the least apprehensive," said Harry Bacas, a reporter for the U.S. Chamber of Commerce newspaper that Lewis edits.

"He seemed satisfied that at last things were going along as they should," Bacas said.

The Lewises' plight was first publicized in late July when President Reagan issued a nationwide appeal for a donor for 16-month-old Candice Thomas of Accoceek, Md., who is now recovering in Pittsburgh and yesterday was reported in fair condition.

At the time, Monica was given less than two months to live by her doctors and was at the top of the hospital's list of children in critical need of a new organ. Last week one of her physicians said Monica probably would die within two weeks without a liver transplant, according to the Lewises.

Zitelli said the liver transplant surgery--a $100,000 procedure that is widely regarded as the most challenging of all transplant operations--would be eased in Monica's case because she had no previous abdominal surgery.