Heating oil leaking from a holding tank in a Northwest residential building oozed down Connecticut Avenue, slid into storm drains and eventually emptied into Rock Creek yesterday morning.

By midmorning, the National Park Service had contained the oil slick with three sausage-shaped absorbent barriers at the point where the oil entered the creek just above the National Zoo at Klingle Road.

For Michael Donahoe, manager of the 3701 Connecticut Avenue Condominiums, his problems began about 7 a.m. when his desk clerk called to tell him oil was flowing down the circular driveway in front of the eight-story, yellow-brick building.

Donahoe, who said he had never faced an oil leak before, did what he thought was right.

"I called the police," he said. "I called the fire department. I called the building engineers. I called the insurance company."

Yesterday afternoon Donahoe watched an oil company pump out the two holding tanks to determine the source of the leak, and an environmental cleanup company shoveled the heavy, black oil from the lawn and sidewalk.

"We don't know what caused the leak," he said. "The were no trucks here filling the tanks. There was nothing unusual."

Donahoe said the oil furnace was a backup for the gas-heated building.

Earl Kittleman, spokesman for the Park Service, said the federal agency was prepared for such accidental spills and immediately put the booms into place in the water near the zoo and as an added precaution placed more booms at the mouth of the creek where it meets the Potomac River at Thompson's Boat House.

"We hope we have contained the oil," Kittleman said. "We know 1,200 gallons leaked out. Over a period of several days all the remaining oil should wash into the creek and then we will start our cleanup."

Kittleman said the agency considered the spill to be serious and compared it to one earlier this year when an oil tank truck flipped over on the Beltway near the Mormon Temple and oil entered the creek.

"At this point we don't know who is to blame," he said. "It is too early to tell. We will have to wait and see."