The Pentagon says it intends to fire manpower executive John C. Tillson, not because he revealed secret information but because, according to formal charges against him, he disclosed "official information to unauthorized persons" and disregarded defense regulations "for the protection of classified information."

Tillson, who denies the charges, is the first government employe known to receive a dismissal notice since President Reagan approved a tightened security directive.

The Pentagon action against Tillson stemmed from an investigation, including lie detector tests, to learn the source of The Washington Post story of Jan. 8 on a private meeting of the Defense Resources Board. The story reported that the board had been told it would take $750 billion more than projected to buy the military forces the Joint Chiefs of Staff said would be needed to rearm American under Reagan's five-year blueprint.

The Pentagon has not challenged the accuracy of the story but says such public disclosure amounts to "giving aid and comfort to adversaries." Here are excerpts from the Tuesday briefing of Henry E. Catto Jr., assistant secretary of defense for public affairs:

Catto: "We have the right to deliberate in confidence. The whole governmental process would fly apart if there were no confidentiality within the deliberating process . . . . "

Q.: "People at the very top regularly leak information, sometimes classified information, to reporters. They've never been punished or prosecuted as far as I know."

Catto: "There are people at some levels, like the president, who have a broad enough picture of the overall effects of the likelihood of what you might call a leak . . . so that he is better able to make a decision like that than a person on a lower level."

Q.: "Henry, you used a phrase giving aid and comfort to adversaries. Who are you talking about, the Soviets or the congressional critics of the budget?"

Catto: "I had in mind the Soviets."