It's been a good life for Dr. Daniel Ruge. Neurosurgery is the top-of-the-line medical career, the "queen of surgical specialties" Ruge calls it, an object of awe and deference within the medical community. It was Daniel Ruge's specialty and he was good at it.

Up until a week or so ago, he had rather regarded his present position as director of Veterans Administration spinal cord injury service as his last job. He is 63.

Or, as Greta Ruge, his wife of 39 years, puts it, "well, at this age one rather expects to be winding down . . . "

Instead, to his still undissembled surprise and undisquised delight, Daniel Ruge (pronounced roog-ee) is going to the White House.

He has been tapped as the new White House physician, replacing the retiring Rear Adm. William Lukash, who is leaving after 14 years and four presidents.

Geographically, the move for Dr. Ruge will be just across Lafayette Square -- at the moment, he's in the VA building at the corner of Vermont Avenue and I Street.

But as he sits, his trim, 6-foot-2 frame folded leggily into his government-issue swivel chair, the long, delicate fingers that demark either surgeon or pianist loosely folded, he contemplates that other world of limousines, Secret Service and presidential planes with a combination of eagerness and uncertainty.

Ruge is lean -- he often walks the two miles from his Georgetown house to the office -- but not especially athletic. His white hair is crisply cut. His eyes are a clear blue.

He is saying that he doesn't really know the Reagans all that well, hasn't seen them for seven or eight years. Of course he's known Nancy Reagan since she was in college . . . he was after all in practice with her father, Loyal Davis. It was Dr. Davis who called him about the White House position.

(Actually Loyal Davis is Nancy Reagan's stepfather, "if you go by blood," says Dr. Ruge, "but as far as I'm concerned, as far as she's concerned, he's her father. I would never call him her stepfather.")

Anyway, that's the connection.

Dr. Loyal Davis, credited by many as the principal force in the Reagan switch to conservatism, was Daniel Ruge's professor at Northwestern University Medical School. In his senior year Ruge became Davis's clerk, eventually becoming his resident and finally his partner in their successful Chicago practice.

Dr. Davis was the first physician in the Chicago area to limit his practice to neurosurgery.

The Davises and the Ruges remained close friends, even after the Reagan in-laws moved to Phoenix, Ariz., and, five years ago, the Ruges to Washington.

("We didn't see them all that much," concedes Greta Ruge, "but we talked a lot on the phone. I always said it's cheaper to talk than to go . . ."

Actually, the connection had already gone another generation: Nancy Reagan's brother Richard Davis also trained in neurosurgery at Northwestern and became Dr. Ruge's resident. He now practices in Philadelphia.

The Ruge's have two children: Charlotte, 29, a student at George Washington University and married "to an ecology-minded lawyer," counsel to a House Interior Subcommittee, and Tom, 26, in Oakland, trying to break into TV or the movies as a director. "He works some of the time," says his father.

"I think they like it," Greta Ruge says of their children's reaction to Dr. Ruge's appointment, "I think it's marvelous that their father can do something like that at this point in his life." She giggles.

"But it certainly is one big surprise to have this very exciting tail-end to your life."

Neither of the Ruges is certain how much Greta Ruge will be involved. Dr. Ruge "rather imagines" he will be traveling with the presidentail party, "but I won't be hovering over them reminding them that something horrible could happen at any moment. I don't view my job that way at all." But whether Greta Ruge will come along is one of the uncertainties. She's planning to watch the inaugural "the best way -- on TV.

In fact, Dr Ruge makes it clear that once he formally makes the move, he will adopt a stance of least possible visibility."

"I'll just tell you this," he says, "the first time I met Dr. Lukash, I told him that the best thing I could say about him was that I hardly knew who he was. I hope I can do as well, and I think he has something he can be very proud of."

Dr. Ruge has already met the staff assembled by Dr. Lukash and he says, has let them know he likes them very much. He plans no immediate changes, but will probably add a couple of M.D.s (one confirming a Lukash selection and one of his own) to the three nurses, a corpsman and physician's assistant already there.

A Republican himself -- "I was one even before the president-elect, and I'm one of the few who doesn't claim credit for converting him either" -- Dr. Ruge is looking forward "to getting to know them better, to being part of the team."

Yes, he conceded, he had rather thought of retiring and devoting more time to his favorite sport, fly-fishing (no golf -- "i'm not very good," he says) along about October 1984.

"Now" -- he grins -- "I'm thinking maybe January 1989. . . ."