A Loudoun County Circuit Court grand jury has indicted a county deputy sheriff on two counts of perjury, alleging that the deputy lied in affidavits he filed for obtaining search warrants in two drug-related cases.

The indictments were the first following a recently completed special county grand jury investigation of sexual and financial improprieties allegedly committed by county Sheriff Donald L. Lacy, a Republican.

The indictments of Sgt. Mark W. Gregory, a six-year veteran of the Loudoun force, were based on the special grand jury's report, which alleged that Gregory gave improper information in obtaining warrants for the searches of two residences in the Sterling Park area of the county.

The first warrant was obtained on Nov. 7, 1980, a spokesman for the Loudoun commonwealth's attorney said yesterday. In the affidavit, the spokesman said, Gregory described in detail a residence where an informant allegedly purchased a quantity of the illegal drug PCP.

Upon further investigation, the spokesman said, the house described in the warrant was not the house where the purchase was made. That warrant was never executed, the spokesman said.

The second warrant, obtained on Feb. 13, 1981, resulted in two arrests on charges of possession of PCP. Those charges were dropped, however, when "statements made in the warrant that are usually easily confirmable could not be confirmed," the spokesman said. The spokesman declined to elaborate on the cases.

Gregory could not be reached for comment yesterday. He is on previously requested annual leave but will be placed on administrative leave without pay when he returns "in a week or so," a Sheriff's Department spokeswoman said.

In its report, the special grand jury alleged that Sheriff Lacy was "aware of, and part of, attempts to cover up" evidence of Gregory's alleged perjury. Lacy could not be reached for comment.

The indictments of Gregory Monday night were among the first handed up by the regular felony county grand jury, which will sit for the next two months.