An early morning fishing excursion ended in apparent tragedy yesterday when two McLean teen-agers slipped off the Little Falls pumping station dam and disappeared into the swirling waters of the Potomac River.

Police and fire officials said those missing and presumed drowned are David Lofton, 17, of 1805 Great Falls St. and Ronnie Cousins, 16, of 1609 Hunting Ave.

According to reports, the two youths were fishing with Cousins' brother Kevin, 18, at 9 a.m. yesterday morning on the Virginia side of the river, directly across from the pumping station.

Lt. Daniel Irving of the Glen Echo Fire Department, that headed the two hour search that involved 20 men and one helicopter, said that the youths apparently waded into the shallow water flowing over the dam.

"When they got out there either one or both of them slipped - we're not really sure, Irvine said. "The dam there is about six feet wide and slippery. People just shouldn't be out there at all.

"When they went in the water the third boy saw them go down and come up a couple times. Then they went down and didn't come up. That's when he ran to call for help."

Search efforts proved fruitless. The current is so swift around the dam that the searchers could not put boats or divers into the water in the area where the youths were last seen.

The pumping station is 1.8 miles west of the Chain Bridge and, according to Irvine always has been considered a hazardous area. Ironically, the north bank of the river is less than 100 yards from the C&O Cannal, provides both good fishing and tranquil water.

Boaters in the area yesterday said that people familiar with the river never went boating or fishing between the bridge and the dam.

"We've had problems up there for years," Irving said. "Something like this happens periodically. We've had people down in that area while wearing lifejackets. There's just no way to judge the current because it can change radically in a matter of feet."

There is little to prevent swimmers, boaters of fishermen from reaching this segment of the river, although there are signs warning people that the immediate area is hazardous.

Glen Echo Fire Chief Joseph Fuller said that once the boys were in the river, they had little or no chance of surviving.

"Ninety nine point nine percent of the time when someone goes in there they never come out," Fuller said.

The Fairfax County Fire Department and the U.S. Park Police remained in charge of the search last night.

The incident immediately followed a warning from D.C. harbor police, who have maintained that no one should be allowed to swim in the Potomac because of water safety considerations.