It's said that Jefferson walked to his inauguration at the Capitol, and we know how Jimmy made his way to the White House. Neither historic event was in mind when I took my own stroll down the Avenue last week. It was merely a perfect October day, a good time to walk in Washington.

There may be grander urban scenes than that from the top of Capitol Hill, but surely few compare to the blending of beautifully kept grounds and distant horizons as you move down into the city. At the foot of the Hill, where Constitution Avenue crosses Pennsylvania, there used to be a fitting symbol of influence in the form of the ESSO Building; it has been replaced by the new Labor Department. Not too bad a structure, either: more modest than many, and somewhat hidden by a screen of shrubbery. Directly across the Avenue something splendid is rising. The new I. M. Pei-designed East Building of the National Gallery is graceful and soft and airy, an inviting combination of stone and glass that makes you eager for its opening next year.

Walking down the north side of the Avenue, nothing seems changed since John Kennedy rode by on his inaugural day 16 years ago. It was then he noticed the shabby, dilapidated buildings and became determined to do something about them. Nothing changed, that is, until you reach the midway point between Capitol and White House.

There, set back from the Avenue and set overwhelming it by the sheer weight of its massive presence, is a collection of buff-colored concrete that looks like a gigantic pillbox set on stilts. Great iron gates that sink deeply into the ground cut off an interior court from public view. The cement itself looks as if it would withstand a Panzer division assault. Brooding over and extending from this mass of ugliness is a rectangular roof deck of the same buff-colored concrete; it provides the final touch, the ultimate bunker effect. Well, not quite final: around the entire building the streets are torn up. They appear to have sunk under the weight of this monstrous structure.

And indeed they have. The J. Edgar Hoover FBI Building, at $129.1 million the most expensive ever constructed in Washington, looks as though its fortress lead-quarters is surrounded by a moral. During construction, the shoring did not hold properly. The streets sank as much as three feet. For almost 10 years there has been litigation between the government - first the United States, and then the District of Columbia - and two of the contractors over damage to the streets. That finally has been settled. Work now is proceeding to bring the streets back up to their level.

But that's not what is so riveting about the FBI Building. What's important is how this atrocity was committed, and what effect it will have on new attempts to transform Pennsylvania Avenue, the nation's main street and most symbolic ceremonial thoroughfare, into something more vital than the deadening assemblage of monumental buildings. For the FBI Building, it turns out, is the linchpin, or dividing segment, of the new Pennsylvania Avenue development plan.

In 1962 John Kennedy, by executive order, provided funds nor a study on redeveloping and designing Pennsylvania Avenue "Rescuing" might be a better description. Ironically, many years later the first tangible fruits of that effort were the opening of the FBI Building in 1975. The Pennsylvania Avenue Commission itself, whose ambitious plans had received national publicity, foundered and went out of business.

Space does not permit the telling here of how that dream was kept alive, nor of the people whose efforts have now brought it closer to reality. But this summer the successor Pennsylvania Avenue Development Corp., created by an act of Congress, received its first federal funds to carry out its mission. The money was not the nearly $300 million it had sought, nor was the asked for 15-year project period granted. Still, the corporation now has some $51 million for design and acquisition of properties, and authority to carry out its plan. And it has a full-time staff hard at work in an office overlooking the Avenue.

Let it be said an air of refreshing excitement and enthusiasm permeates this operation. Its leaders are not some jaded types tearful of expressing their emotions - or of their dreams. They talk of saving historic old buildings including the Willard Hotel, and remodeling them: of creating a strip of federal-style facades: of fostering new and innovative development, with expensive shops and restaurants. And most important of getting away from the stuffiness of the past.

"Within five years I think it's realistic we'll be looking out the window and seeing a complete rebuilt Avenue itself," say David Harris, the finance director.

"From Third Street to Fifteenth Street you'll see two rows of excellent, large-sized healthy trees on the south side of the Avenue and three rows on the north. They will be stretching along with entirely new pavement, new signs, new signals, elegant new lights. It may not be the Champs Elysees, but it will be an American variant of it - a really high-quality, ceremorial avenue."

His boss, the new executive director, W. Anderson Barnes, recognizes that good design, no matter how attractive, will not be enough. The question is how to bring life to the Avenue. "When you think about it," he says, "there are only a few great boulevards in the world and they all have a common element - they are bustling, truly bustling night and day with all kinds of people: rich and poor, old and young.So to succeed it's very much our mission to find out how to bring all those different kinds of people there with a purpose and to have the amenities that are attractive to them."

But, then, you will stumble on that monstrosity in the middle of the Avenue, the FBI Building.

"No element of our national government, perhaps not the government at the time that building was conceived and designed and built had control over the gentleman who was a very major character in American life," Dave Harris says, looking back on how it happened. "And at that time there was no body with a guiding hand over the development of the downtown. Even it we had been there it would have been very difficult. No one was watching, so to speak this very estimable character in American history who had his own designs and ideas about that building. It's there, and it's a given."

The government agency that has most direct responsibility over that building - the General Services Administration - views it from a different perspective: it was a lesson to be learned, James Stewart, GSA's overseer of all federal construction in this region, running to some half a billion dollars at present, puts in this way:

"If you talk about the FBI and how could that building happen, I think you have to go back and look at the process and look at what was going on in architecture in the late '50s and early '60s. This building was the product of the end of this very neo-brutal architecture. At that time the whole scheme of things was to design a building unto itself. I think that's the end of an era. We've learned from that to humanize things a lot more. We're much more into the human spirit - how does it fit with its neighbors in the community, does it invite you in and make you feel you're a part of what's going on in this town."

"And that building in fact is, and always was, a monument to J. Edgar Hoover. He had a very strong influence on what was being built. They in fact were developing an image of saying. 'This is the FBI.' And if you look at it very closely as coming out of J. Edgar Hoover you can almost see it. There is is."

And there, since it will not sink, it stays. Both the GSA and the Pennsylvania Avenue Corp. have talked about ways of possibly softening that imposing exterior - of alleviating what Dave Harris calls "its almost malevolent presence."

No final solutions yet, but someone on the Hill has a suggestion: cover it over with three layers of ivy. Then it could be the first green parapet on the Avenue.