NOW THAT Washington has begun to grasp the shape of the new Reagan Cabinet, it is time to begin thinking about where most of the real power will reside in the new administration.

No, it will not be an official or unofficial "supercabinet," a notion of some close Reagan advisers who would like to be super-cabineteers. Nor, for that matter, will it be with the Cabinet as a whole.

If experience is any guide, we will have under Ronald Reagan what we have long had in Washington -- subcabinet government. Outside of urgent crisis areas, foreign and domestic, most power will rest with the legion of undersecretaries, assistant secretaries, general counsels and their staff who comprise the subcabinet.

That is why the Reagan people are wise to have prepared lists of prospective subcabinet members for their new bosses, who will have to choose from among them or else get special permission from the White House command to hire somebody else. Having as tight control as possible over the subcabinet is no sin; it is essential to the success of any administration, regardless of whether career bureaucrats, the press or others sometimes portray it as a sinister scheme (in Washington, of course, anything Richard Nixon once tried to do is ipso facto "sinister").

Indeed, many of the outgoing administration's problems stemmed from the weakness of Carter's subcabinet -- which was generally chosen by Cabinet members themselves, with little White House consultation. The unpopularity of the CETA jobs programs, to cite just one example, is traceable directly to the administrative weakness of Ernest Green, the assistant labor secretary responsible for it.

The White House and Cabinet members, after all, spend their time dealing almost exclusively with issues that are on the front pages. The subcabinet, meanwhile, controls what most voters consider "the government." They manage the federal departments and their myriad programs, develop new policies, legislation and regulations, dicker with Capitol Hill and the inescapable constituency groups. All this brings with it control of perhaps the most important resource in government -- information.

For example, Donald Regan, whatever his merits as treasury secretary, will not come into government with a detailed roadmap of the massive maze that is our tax code. Sure, he may have learned something about "commodity straddles" and other questions at Merrill Lynch, but that won't get him very far if he tries to tangle single-handedly with wuch Senate powers as Robert Dole, Russell Long and the staff of the Senate Finance Committee.

You can bet that the key player in the tax policy game will be an assistant treasury secretary whom nobody below the 60 percent tax bracket has ever heard of. When the final tax package is being polished in the White House, he will be the guy at the table with the tallest mound of computer printouts. His power will come from saying something like, "That's a very intriguing proposal, Mr. Meese, but have you considered the 11 unintended consequences that charge would have on overseas sales, to say nothing of the balance of payments, in the third and fourth years?"

During the Carter years the person who cut the deals on tax policy was Gene Godley, the assistant treasury secretary for legislation. You may not have known who he was, but Don Regan at Merrill Lynch probably did. Godley, of course, had his battles with the White House, but the president's men there ended up respecting his expertise and authority as he shepherded tax legislation through Congress for Michael Blumenthal and G. William Miller.

In the same way, Ray Donovan, Reagan's choice for secretary of labor, undoubtedly has some strong negative feelings about the Occupational Safety and Health Act (OSHA,) but translating those antagonisms into national policy can't be done without the active complicity of the assistant secretary for OSHA.

Carter's White House economists also wanted to soften new OSHA regulations, but even those economists -- who devote their lives to mind-numbing details -- were no match for the OSHA technicians, who beat them down with mounds of scientific data on things like cotton dust standards in textile plants. As a result, Eula Bingham, the current assistant secretary for OSHA, and Basil Whiting, her deputy, won far more of those battles than they lost.

Godley at Treasury or Whiting at Labor are by no means alone. Robert Embry, an assistant HUD secretary, both invented and ran the largely successful Urban Development Action Grants program. Robert Hall, a career bureacrat who became an assistant secretary of commerce, had a large degree of automony in doling out billions of dollars under the economic Development Act. The list goes on, as it will under the Reagan administration.

Elliot Richardson, the nation's only professional secretary-of-something-or-other, says that "it is self-evident that a Cabinet member doesn't run a Cabinet department by himself." Selecting the Subcabinet "is the most important thing he can do," he adds.

Even if a cabinet secretary wanted to run everything in his department, the external demands on him would make it impossible. There are the time-consuming meetings in the Roosevelt Room of the White House and the repetitive congressional hearings, preceded by hours of briefings and courtesy calls on Capitol Hill.

Add to this the steady diet of politically necessary out-of-town speeches before important constituency groups, a reality which often will mean that, on a given day, half the Reagan cabinet will be changing planes at O'Hare Airport.

And don't forget the ceremonial functions within a bacinet department. A new secretary also will quickly be convinced that morale among the career bureaucracy will be destroyed unless he visits all the regional offices in the first six months.

As Richardson remarks, carefully delegating responsibility is "a given" for a Cabinet officer. Sometimes the delegation is handled smoothly, as it was at HEW when Joseph Califano would have a daily lunch with Hale Champion, his undersecretary. But sometimes a power vacuum at the top creates unwieldy bureaucratic arrangments that dramatically enhance the power of the subcabinet, as it did when David Mathews had the title of HEW secretary at the tail end of the Ford administration.

Mathews is the best recent example of a cipher in the Cabinet. As one-high ranking veteran of the Ford White House put it, "We sent Mathews to HEW and never heard from him again. He used to show up at Cabinet meetings every three or four weeks but nobody was ever sure why."

The undersecretary was seriously ill with cancer, so Robert Morrill, the little-known assistant secretary for policy, took the lead in holding the vast department together. He was the one who handled the negotiations with the Domestic Council in the White House and congressional committees. He didn't get much publicity, but for more than a year, with the assistance of HEW's comptroller, he exercised the powers of a Cabinet secretary.

Nobody better understands the importance of the number two man in Cabinet-level agencies than Frank Carlucci, the deputy director of the CIA, who may become the Elliot Richardson of the subcabinet.

He was Casper Weinberger's undersecrtary at HEW in the early 1970s and is a leading, albeit controversial, candidate to follow his former boss to Defense as the deputy secretary. It is a post that John Marsh, a former Ford aide mentioned as a possible secretary of the Army, calls "the most important job in the subcabinet." Traditionally, the defense secretary worries about global strategic policy and the deputy secretary handles the nuts-and-bolts of running the Pentagon.

Carlucci's career at the CIA suggests that in the foreign policy/national security arena even the number-two man may not have the time to actually run the agency. Long, agonizing hours are spent at the White House managing the foreign policy crisis of the moment. A tremendous amount of coordination is required between Carlucci and people like David Newson and David Aaron, his counterparts at State and the National Security Council. Even moving from HEW to the CIA did not free Carlucci from the necessity of congressional hearings. The only difference is that, these days, his testimony takes place behind closed doors.

The same phenomenom of top policymakers being overwhelmed by the immediate evidently takes place at State as well. "What you have to understand is the top people at the State Department tend to be preoccupied with the crisis of the moment -- the Iranian hostages, relations with the Soviet Union or whatever," says Matthew Nimetz, who has just left the post of undersecretary of state for security assistance. "That means there has to be a tremendous delegation of authority."

Naturally, delegation of authority can only work if those in the subcabinet are capable of receiving it. That's why the Reagan team is totally correct is trying to carefully control the staffing of Cabinet agencies. But their efforts will be for naught if they simply select the subcabinet on the narrow criteria of political loyatly and ideological suitability.

Even if they succeed in eliminating all closet liberals, they will still have to avoid three other types of weak assistant secretaries who keep cropping up in Washington:

The Runnaway Egotist. This is the type who is more concerned with speech invitations, press clippings, presiding majestically over unfocused but well catered meetings -- in fact, listening to himself talk in general -- than with actually running a program.

The Bored Generalist. This is the person who is shocked to discover that being an assistant secretary of commerce does not automatically qualify you for "A List" parties. He and his wife quickly go into a four-year pout and he is content to let his career deputy run his agency.

The Unqualified Bag-Carriers and Tokens. These are the types who got jobs one notch higher than they deserved because of race, sex or yeoman service in the presidential campaign. They are distinguished by lack of interest in any memo longer than three sentences. If you have to hire them at all, put them in charge of other bag-carriers or tokens.

The message is clear: the Reagan team should be looking for more than competent yes-men in filing the subcabinet. They should want people as managerial talent and general loyalty to the Reagan administration.

They are the people, after all, who will be running the government. As Ben Heineman, a former HEW assistant secretary, says, "The subcabinet members are the grunts of government, working 14-hour days to make the machinery run a little less badly."

If the subcabinet members are good, they will shape the lasting legacy of the Reagan years. And, if not, life will go on as always and those in Washington hoping for far-reaching change will curse their naivete.