District of Columbia police officer Thomas K. Delahanty underwent successful emergency surgery at Washington Hospital Center last night to remove a bullet lodged in his neck after the FBI concluded that alleged assailant John W. Hinckley Jr. may have been firing explosive "Devastator" bullets at President Reagan.

FBI spokesman Roger Young said the decision to operate on Delahanty -- one of three men wounded with Reagan in Monday's attack -- was made because it was possible that an explosive charge in the slug of the 22-caliber bullet was unstable and could detonate.

A Washington Hospital Center spokesman said last night that Delahanty agreed to the surgery and that two volunteer doctors, reportedly wearing flak jackets, operated on him in an isolated room.

Delahanty was in "serious but stable" condition early today after about three hours of surgery during which the bullet was removed, a hospital spokesman said.

The bullet was turned over to the FBI, but there was no immediate information as to whether it was a "Devastator." An FBI spokesman said the bullet would be examined this morning.

Authorities began considering the possibility that the rare bullets were used in Hinckley's alleged attack on the president when an empty cartridge box with the "Devastator" label was found in Hinckley's hotel room Monday afternoon.

Late yesterday afternoon, FBI agents found in the car fragments of the bullet that hit the president's limousine window. Delanty's doctors were told of the increased possibility that he had been hit by an explosive bullet, and surgery was ordered, Young said.

Four fragments of a bullet had been removed Monday from the brain of presidential press secretary James S. Brady, Young said.

The bullet that hit Reagan in the chest ricocheted off his limousine but did not explode, and will be examined today to determine if it, too, is a "Devastator." Young said. He said it is possible that all six of the bullets fired were the exploding "Devastators."

It is not known why some of the bullets allegedly fired by Hinckley apparently exploded while others did not. For instance, the bullet that hit Brady may have exploded when it struck his skull, Young said. Others may have malfunctioned, he said.

The bullet is designed to explode and fragment on impact. It is made by Bingham Ltd., a firm in Norcoss, Ga., by putting an aluminum cap filled with explosive into the nose of a normal .22-cal. long rifle cartridge. The cartridges cost $7.12 for a box of 12, compared with a top retail price of about $2.40 for a box of 50 standard cartridges.

Young said it is not known yet where Hinckley could have purchased the deadly cartridges or how many he bought. An official of the Bingham company told a Washington Post reporter recently that his firm has been making the "Devastators" since 1978 and sells them to about 3,000 customers nationwide.

They were originally made for use by police, he said, and because of the explosive power they have the impact of a much larger bullet, such as 9mm or .38 cal.

George Kass, a Michigan expert and police consultant on the market. A District police firearms expert said he had never encountered one of the explosive bullets.

An advertisement for the "Devastator" shows a picture of a shattered Coke can with the printed claim that "our .22-cal. Devastator" inflicted the damage from a distance of 50 feet, Young said.

Richard M. Loughery, chief executive officer at Washington Hospital Center, briefed reporters about 10:20 last night, saying the FBI had told the hospital late in the afternoon that "there was a very good possibility" that the bullet lodged in Delahanty's neck "could be an impact-sensitive explosive bullet. It was determined it should be removed as expeditiously as possible."

Delahanty and his wife were told about 5 p.m. that emergency surgery was necessary. Delahanty had expected to be sent home today but now is expected to be hospitalized for another week.

Dr. Michael Dennis performed the surgery, assisted by Dr. Norman Horowitz. Both are neurosurgeons.

"As long as we didn't hit it [the bullet] with a chisel or drop it on the floor or something radical like that, we were told we probably didn't have to worry," said Dr. Howard Champion, director of the hospital's shock-trauma unit who observed the operation.

The bullet was lodged near Delahanty's spinal column, and there had been no plan to remove it until news came that it might be explosive.

Delahanty had been listed in good condition and steadily improving earlier in the day.

Brady remained in critical condition in the intensive-care unit at George Washington University Hospital, but doctors said he was clearer mentally and able to move the upper part of his left arm and leg slightly in response to commands.

Asked how he felt, Brady gave a thumbs-up sign and said, "fine, fine," according to a White House statement last night. The response and movements encouraged his physicians, since they are concerned about whether his brain injuries will affect his speech or muscular control on the left side of his body.

"We are a long way from home on Mr. Brady," said Dr. Dennis O'Leary, a hospital spokesman. Drainage tubes placed in Brady's head during surgery have been removed, he said, adding that Brady's eyes are still swollen shut, but he had correctly counted fingers a doctor held up before him when his eyes were held open.

The White House reported that Secret Service agent Timothy J. McCarthy was in good condition at George Washington University Hospital, but was experiencing some soreness from his operation. A doctor familiar with the case said McCarthy may be in the hospital longer than the president because he has tubes draining fluid from both his liver and right lung.