The White House, on the heels of revelations that five employees were relieved of their duties as authorities investigate drug use, said today it will soon begin mandatory, random drug tests. Spokesman Marlin Fitzwater said the months-old investigation, which has resulted in the dismissal of two National Security Council clerks and the suspension of three White House guards, ``illustrates all too well the terrible fact of drug use in our society.'' Reagan, asked at a picture-taking session about the developments, first reported in today's Washington Post, said, ``Yes, of course I am upset that it (drug use) is found anywhere.'' Asked if he thought his security had ever been endangered, the president replied, ``No, I don't think so.'' Reagan, posing with Republican congressmen who had come to the White House to discuss drug policy options, said that if White House workers are found to have used drugs he would like to see them enter drug treatment programs. ``If it was just a case of using'' drugs, Reagan said, ``then I would like to see us do our best to get them into a drug-treatment organization with a view to attempting a cure. ... Let the people know that then we will do our best to salvage anyone who has been a victim.'' On Wednesday, White House spokeswoman Liz Murphy said the two clerks had ``admitted drug use and were separated from the NSC.'' Fitzwater today told reporters that the clerks ``agreed to resign'' several months ago, ``during the winter.'' Asked at what level the NSC clerks worked, Fitzwater said he could not specify except to say they were at the ``secretary level.'' Pressed to say whether the two were involved in handling sensitive matters, he responded, ``It's fair to assume that everyone who works at the NSC has access to sensitive material.'' Fitzwater said Reagan and first lady Nancy Reagan, who has led a campaign to ``just say no to drugs,'' have known of the investigation by the Drug Enforcement Administration and the Secret Service ``for several months. ``It was started last winter when allegations were brought to the attention of the Secret Service,'' Fitzwater said. He said the tip had come from a member of the Uniformed Division, whose officers guard the Executive Mansion. But the White House later today corrected Fitzwater's statement, saying the information had come from outside the Secret Service. Fitzwater would not say today what drugs had been involved. The Post, however, quoted an unnamed source as saying that a transaction involving cocaine had taken place on the White House grounds. Fitzwater said today there was no evidence produced by the investigation so far to substantiate that any such drug sales occurred. Fitzwater said that White House employees will be sent a letter giving them 60 days' notice of the mandatory, random drug testing. Fitzwater said the tests will be conducted six times a year, with each round involving about 2 percent of the approximately 1,600 employees of the executive office of the president. Included are employees of the Office of Management and Budget, the National Security Council and the office of the vice president and first lady. The spokesman also said that the Secret Service will begin on July 13 a program of pre-employment drug testing for job applicants. ``We have been advised that drugs were not used or sold on the White House premises and there was no breach of security,'' he said, although Fitzwater cautioned that the investigation by the U.S. attorney's office, Secret Service and DEA was continuing. Names of the five employees were not released. Evidence in the investigation, begun several months ago, has been turned over to the U.S. attorney's office, Secret Service spokesman Robert Snow said Wednesday. ``No arrests have been made,'' Snow said. ``These are only administrative actions at this point.'' Two of the guards were relieved of their duties last Friday. The third was placed on leave Monday. It was not clear when the NSC clerks were fired. Asked why the clerks were no longer on the payroll while the Secret Service officers were on paid leave, Fitzwater said the guards have the protection of civil service appeal processes while members of the executive office serve at the pleasure of the president. In addition to guarding the White House grounds, uniformed members of the Secret Service guard the vice president's home and foreign embassies. They also travel with the president to provide security. The NSC, chaired by the president, advises the president on matters of national security.