An investigation yesterday substantiated charges that interns at the National Endowment for the Humanities were being improperly detailed on official time to make arrangements for a birthdayy party tonight for endowment chairman Joseph E. Duffey, according to John Whitelaw, deputy director for management. Whitelaw said that seven college-aged interns had volunteered to perform such duties as writing invitations to about 200 guests and stamping the envelopes. They had done so at the behest of Kay Elliott, the endowment's special assistant for public affairs. Elliott had agreed to assist Duffey's wife, presidential assistant Anne Wexler, in organizing the 46th anniversary party, which was to have been a surprise until publication of press reports about the use of government personnel and time.

"Kay started it as a volunteer project," said Whitelaw, who was designated by Duffey to investigate the charges. "Anne had asked her originally to help find outside aid, but that didn't work out, and Kay volunteered herself. With her duties at the White House Anne just didn't have the time to do it, everybody agrees.

"It is a fact that some of this was done on government time - telephone calls, addressing letters, and things like that. It should not have been done that way, and we are trying to get a precise accounting of how much work has done on government time. Getting an exact amount is important, because Anne and Joe, who knew nothing about this, will pay for all of this work on the party out of their pockets."

Whitelaw reflected the sympathy that Duffey had expressed the night before for Elliott despite what both regard as a mistake on her part."Kay is trying to pull this place together. She puts incredible demands upon herself. And sometimes she puts unintentionally excessive demands upon others."

Elliott recently was named a GS-15 with a salary of $36,171. Asked yesterday if she felt her job was in danger, Elliott said it was not. "Everybody here thinks the flap is pretty much of a joke." And there seemed to be general agreement among officials at the endowment that she would not lose her job despite Duffey's earlier description of the action as "a colossal mistake."

The story broke late Thursday night when Newhouse News Service reporter Susan Fogg quoted discontented staff members in Elliott's office. Their complaints ranged from having to work on the Duffey surprise party to doing personal chores Elliott sometimes asked them to perform for her.

Yesterday there was much speculation about who may have leaked the story but nobody was sure who it was.

According to endowment staff member David Braaten, a friend of Fogg's, Elliott took him aside when he arrived at work and told him: I'd like to know how Susan Fogg got that story." Braaten said he did not ask Fogg, "and she didn't tell me but I suggested that so many people knew what was going on, it probably hadn't been difficult for her to find out."

Invitations went out to many in Washington's news community and included an RSVP telephone number inside the White House. An obvious place to start, according to Braaten, was by calling the White House number which turned out to be Wexler's office.

The party, to be held around the pool of the Duffey's Northwest Washington apartment building, will go on as planned - except for the surprise.

"It's going to be a nifty party," said Elliott, who was sorry that news had leaked out of the posters which depict Duffey as "King Duff." "Can you imagine Joe Duffey's face in a King Tut headress?" she exclaimed.

Said Whitelaw, I'll certainly be there, and the way things have been going they may end up making me the bartender."