Maryland Lt. Gov. Blair Lee III, stressing that he did not want it appear that he was "grasping for power," announced today a delicate arrangement in which he will assume many of Gov. Marvin Mandel's routine duties while the governor recovers from his illness.

Under the arrangement, Mandel retains all of his offical policy making powers, but is turning over to Lee enough of the day-to-day duties of governing "to cover our current needs," Lee said at a press conference here.

The move was abviously a sensitive one, Mandel, a proud man and a master politican, was reluctant to relinquish power while hospitalized. Lee, an announced candidate to succeed Mandel as governor in 1978, likewise did not want to appear to be pushing the ailing governor too far for his own personal advantage.

Yet both men realized that Mandel is unable to perform many of his duties from his bed at Prince George's General Hospital. So, on his 57th birthday, as members of his family gathered for a small party with a birthday cake in his hospital room, Mandel turned over many of the routine functions of his office to Lee without giving up his title as governor.

The arrangement was hammered out in a 75-minute meeting in Mandel's hospital room yesterday. Lee said Mandel appeared to be "a little blue, depressed" when Lee and Alan Wilner, the governor's chief legislative officer, entered his ninth floor, corner room.

But as the meeting wore on and the talk turned to politics "his morale greatly improved, he was laughing and cheerful," Lee added. Later, he said the governor's wife, Jeanne, who sat in the room during the meeting, told him and Wilner that "we were good therapy."

While Mandel "tired toward the end," his mind was "functioning beautifully . . . he was as sharp as he could be," Lee told the press briefing at the State House.

The doctors who have been examining Mandel have scheduled a meeting for Wednesday at which time they discuss whether Mandel should be transferred back to the governor's mansion in Annapolis, remain in Prince George's General Hospital or taken to Johns Hopkins Hospital in Baltimore for specialized tests.

A security officer at Johns Hopkins said he was advised to expect Mandel's arrival, possibly Wednesday at the hospital. The meeting was the first between Mandel and the state's second-ranking official since the governor was hospitalized April 5, eight days before his second political corruption trial was scheduled to begin in Baltimore. A federal judge last week delayed the trial one month after six doctors testified that Mandel's condition made it impossible for him to stand trial at that time.

Wilner negotiated the fine points of the Lee-Mandel arrangement at an earlier meeting with the governor. As he explained it, Mandel simply wanted to give up some of his "paper-shuffling" duties. "There's a big difference between making ultimate policy decisions, which Mandel can do, and delegating the implementation of policy," Wilner said.

For routine purposes, however, Lee will act as governor until Mandel recovers. "My duties will be pretty much . . . what he would do if he were here," Lee said.

He defended Mandel's reluctance to turn over more authority, saying, it was "a natural reluctance for someone who has worked all those years to get it."

The lieutenant governor shares some of that reluctance. "I have a personnal problem in that nobody is going to put me in the position where I appear to be grasping at power," he said at one point. Minutes later, he added, "I do not feel strongly about not being presumptuous, or appearing presumptuous."

Lee said he feels "no need for" additional power right now, although he added that if physicians order the governor to "literally get away" from his public duties, "he (Mandel) would respond accordingly."

The next report on the governor's somewhat uncertain condition will be announced Wednesday afternoon. Preliminary studies have suggested that Mandel is apparently suffering from some sort of a brain lesion.

Lee's new duties were spelled out on two leters signed by Mandel and released today. One authorizes Lee to preside as "acting governor" over Friday's meeting of the Board of Public Works: the other delegates Lee to "perform in your capacity as lieutenant governor to the same extent and effect as I would perform as governor" five general duties.

The letter authorized Lee to deal with federal grants and programs, transfer funds within departmental budgets, assent to gifts to the state, handle criminal extraditions, and make executive agreements "where the approval of the governor personally is not constitutionally required."

Mandel also agreed to meet on a semiregular basis with Lee and Wilner, and has set up a meeting with State Senate President Steny Hoyer and House of Delegates speaker John Hanson Briscoe on Thursday. Until this week, Mandel has communicated with Lee and aides only through messages delivered to him by his wife and state troopers.

Lee came away from his first meeting with Mandel as puzzled as ever about the extent to the governor's illness. "I don't know if he is recovering from a strike (as first suspected by Mandel's personal physician, Dr. Perry Hookman) or if the illness is getting progressively worse," a possibility raised by a specialist who examined Mandel at the request of U.S. District Court Judge Robert L. Taylor.

Lee said Mandel "talks wisefully of getting home this weekend." It is because Mandel expresses such optimism that Lee apparently is reluctant to seek greater authority. He said that Wilner "dumped a pile of bills yayhigh (indicating six inches deep) on him and he seemed to be looking forward" to reviewing them for his signature or veto.