Christopher F. Reckmeyer II, 33, described by prosecutors as the mastermind of a nationwide drug operation centered in Loudoun County that generated more than $100 million in sales over a decade, pleaded guilty yesterday, the day after his younger brother Robert agreed to testify against him.

Both brothers were among the 26 defendants in the drug case and both were given the same assurance that their sentences will be 10 to 20 years in prison, without being considered for parole. The brothers are jailed without bond pending sentencing in U.S. District Court in Alexandria. Christopher is to be sentenced May 10, and Robert April 12.

Christopher Reckmeyer's attorneys would not say what part, if any, his brother's agreement to testify against him played in his decision to plead guilty to conducting a criminal enterprise and two tax law violations. The brother, Robert B. Reckmeyer, 30, accused of being another of the drug ringleaders, entered guilty pleas Wednesday in a plea bargain agreement.

Bernard S. Bailor, one of the attorneys, would say only: "When the dice are loaded you don't play craps and when the deck is stacked you don't play poker."

Christopher Reckmeyer, who ran the marijuana and hashish distribution ring from his 1,000-acre Loudoun County estate, disputed many details of the government's case against him yesterday when questioned by U.S. District Judge James C. Cacheris.

Wearing a rumpled gray suit and frowning frequently, he said that while he made "millions of dollars" from the drug operation, the prosecutors' contention that the total was $13.7 million was inaccurate.

When the judge asked him for a "ballpark figure" of the money he had made, Reckmeyer hesitated for a moment, then replied: "Maybe one-third of that."

He also disputed the government's assertion that he distributed more than 169 tons of marijuana. "I was involved with the distribution of many tons of marijuana," he said. "It's hard to remember back to 1972 right at this moment."

In the end, he told the judge, "I'm quite happy pleading guilty to these charges . . . . I appreciate your indulgence."

Most of those in the public gallery during the two-hour proceedings were government officials who had worked on the case for almost two years. They included agents from the Internal Revenue Service and the U.S. Customs Service and a contingent from the U.S. attorney's office, including the office's receptionist. Their work culminated in indictments returned Jan. 9.

In the crowd sat Reckmeyer's wife, Nancy, who is pregnant with their fourth child, and one of their children. One of Reckmeyer's sisters, Tina, also was in court. His brother Robert was not present.

Part of Christopher Reckmeyer's plea agreement with the government was read into the record at a bench conference and remains sealed. Assistant U.S. Attorney Joseph Aronica declined to say what it said.

As part of his agreement, he agreed to help the government recover all assets, "both domestic and foreign," acquired through drug sales.

The government agreed not to oppose his request that he serve his sentence at a minimum security prison.

Salvatore Vastola and James S. Adams also pleaded guilty yesterday to conspiring to distribute drugs in connection with the operation.

All 15 defendants who were arrested after their indictment have now entered guilty pleas. Eleven other persons who were indicted are still at large.