Two men died and third was listed in serious condition at Howard University Hospital last night after suffering heroin overdoses in separate locations in Northwest Washington, D.C. police said.

A fourth man also was taken to Howard for treatment of a heroin overdose but left the hospital shortly afterward, authorities said.

Police said they had no evidence linking any of the drug incidents, which appeared to be the most serious outbreak of heroin overdoses in the city since March, when a highly potent batch of the narcotic killed nine persons and left at least 14 hospitalized.

Authorities said that all four persons involved in yesterday's outbreak were taken by ambulance to Howard University Hospital between 3:30 and 7:30 p.m.

One of dead men was identified as Percy Watts, 47, of 601 Four Mile Rd., Alexandria. Police said Watts apparently had injected heroin at a shooting gallery and prostitution parlor in the 1200 block of Eighth Street NW. The other man was identified as Mark Duncan, 35, of 5783 Suitland Rd., Suitland. Police said a passerby found Duncan unconscious in a car parked in front of 1205 W St. NW.

An unidentified 32-year-old man from Colonial Beach, Va., was in serious condition last night. Police said a passerby found him unconscious in a van parked in front of 1234 Ninth St. NW.

Hospital assistant administrator Michael Browne said that the fourth man who left the hospital after being treated, a 29-year-old District resident, was transported from the area of M and North Capitol streets.

Homicide and narcotics detectives investigating the deaths said last night they were still trying to determine where the overdose victims obtained the heroin.