Robert B. Reckmeyer, 30, one of the two McLean brothers accused of masterminding a nationwide drug operation, agreed yesterday to testify against his brother Christopher in exchange for a 10 to 20 year prison sentence. He would have to spend at least 10 years in prison before being considered for parole.

Christopher F. Reckmeyer, 33, the owner of Shelburne Glebe, a 1,000-acre estate in Loudoun County believed to be the center of an operation that generated more than $100 million in drug sales over a decade, is scheduled to stand trial Monday. He is jailed without bond and was not in the U.S. District courtroom where his brother appeared yesterday.

Federal prosecutors allege that he personally made $11 million from drug dealing and have described him as "the man on top of the heap" in the alleged operation.

Robert Reckmeyer, who has been jailed without bond since Jan. 15, appeared pale during his court appearance in which he pleaded guilty to one charge each of firearms, tax and currency violations plus conducting a criminal enterprise. He could have been sentenced to life imprisonment without chance of parole for the last offense.

U.S. District Judge James C. Cacheris asked him, "Do you understand you can spend the next 20 years in prison?" Reckmeyer, standing with his hands behind his back, looked toward his wife, Patricia, 29, the mother of their three children, and said in a low voice, "I do, Your Honor."

Patricia Reckmeyer pleaded guilty a few minutes later to one tax charge in the case in exchange for a probationary sentence.

When the judge asked Robert Reckmeyer if he had earned the $9 million in personal profits from the drug ring that the prosecution alleged, his attorney, John Dowd, replied that his client's gain was "actually 4 million."

In exchange for Robert Reckmeyer's guilty pleas and the promise of his testimony at all trials related to the drug case in which 26 persons were indicted, the government dropped 20 other charges against him.

The Reckmeyer brothers and the other defendants were indicted Jan. 9 in an alleged operation that prosecutors say distributed more than 293 tons of drugs -- marijuana smuggled from Colombia and hashish smuggled from Lebanon.

A total of 11 defendants in the case pleaded guilty yesterday in the case and agreed to cooperate with the government in recovering assets generated by the ring's alleged $100 million drug sales.

Robert Reckmeyer and the others who pleaded guilty will be sentenced April 12.

Assistant U.S. Attorney Karen P. Tandy declined to assess what impact Robert Reckmeyer's testimony might have in the case against his brother, who faces 14 drug and tax charges including the charge of conducting a criminal enterprise.

Two others arrested in the case, Gregory A. Jarrett and Salvatore F. Vastola -- the only other defendants in custody who have not entered guilty pleas -- are scheduled to stand trial with Christopher Reckmeyer on Monday. Twelve of the 26 persons under indictment are still at large, including the third alleged kingpin, Bruce W. Thomason of Waldorf, Md.

Tandy said the ring "laundered" its profits through businesses that sold precious gems and oriental rugs. Robert Reckmeyer agreed with Tandy's statement yesterday that in 1980 he paid for a shipment of drugs with two oriental rugs, a silk rug, and 2,000 ounces of silver coins.

He also agreed that on a separate occasion in 1980, he paid for 13,000 pounds of hashish delivered to Hilton Head, S.C., with $1.88 million in cash and a 6.08 carat ruby.

Those who pleaded guilty yesterday to charges of conspiring to distribute drugs were:

David W. Cook, 29, of Alexandria; Richard F. Vaughn, 51, of Baltimore; Patrick R. Hendry, 34, of Lovettsville, Va.; John Christopher Robertson, 30, address unknown; Donald Shirack, 62, of Tucson, Ariz.; Jan R. Gillie, 36, of Blacksburg, Va.; Francoise Nasta, 28, of Woodbridge; Thomas W. Ashton, 30, of Bluemont, Va., and Steven S. Johnson, 23, address unknown.