John C. Apperson, an aide to Rep. Ronald V. Dellums (D-Calif.), testified for more than three hours yesterday to a federal grand jury investigating alleged drug use on Capitol Hill.

Apperson was called to testify before the grand jury in connection with allegations made by Robert T. Yesh, a former employe in the House doorkeeper's office, who told investigators that he sold cocaine and marijuana to Dellums and that Apperson allegedly served as a go-between for the seven-term congressman. Yesh pleaded guilty to drug charges last March.

Both Dellums and Apperson have denied the allegations and sources say Apperson denied those allegations yesterday under intense cross-examination by federal prosecutors before the grand jury. Sources close to the investigation say Yesh's testimony is key to any government case against Dellums and that strong denials from Apperson, who testified under a grant of limited immunity, could be a major blow to that case.

At one point during the closed grand jury session prosecutors played a tape recording of a conversation between Apperson and Yesh, who had been cooperating with investigators. Sources say the conversation included some discussion of drugs but it was not known yesterday how Apperson responded to questions about the conversation.

Apperson emerged from the grand jury room about six times during the three-hour and 15-minute session to consult with his attorney, Michael Kennedy. Both Apperson and Kennedy declined comment afterward.

Dellums' attorneys, Samuel J. Buffone and Austin J. Doyle Jr., also waited outside the grand jury room during the questioning of Apperson.