SOME OF US, when we leave Pennsylvania Avenue after dark, will be feeling pleasantly surprised by the quality of nightlife there. Others of us will leave feeling somewhat unfulfilled or disappointed. The point, however, is that all of us, no matter what our impressions while we're there, will leave.

Because we don't live there.

Because while the Avenue of the Presidents is a nice place to visit, even at night nowadays, almost nobody lives there but presidents. And the White House, though it does have a bowling alley and a good deal of live-in help, does not generate a lot of spontaneous, continuous foot traffic.

Nobody at 1600 Pennsylvania, for instance, goes out for a sudden pizza at 11 p.m. on a Friday, like some of us used to do when we lived near Dupont Circle (and were, admittedly, about 10 pounds heavier).

Instead, at 9 o'clock on a recent Thursday night, you might find three men sitting at the otherwise barren bar of J.J. MelBFlon's, which is across 13th Street from the new National Theater and across the lobby of 1201 Pennsylvania from the new Bread Oven restaurant/club. Of the three patrons, one is there to investigate nightlife; he is thus hunched over his second $2.25 chablis. Pay no attention to him.

The other two, however, apparently are from the architectural firm that designed the building and maintains an office in it; they seem significantly more cheerful than, let's say, the guy sitting alone in the shadows near the door.

That would be the owner. Probably brooding about the cost of overly wide sidewalks, or possibly the rent. Or the interrelationship thereof.

Okay, but listen -- you can help this guy (actually Joe Malhotra, ex-Joe & Moe's) and his nearby peers. Until the Pennsylvania Avenue Development Corporation gives us someplace dow here where we can raise families -- or just the bedroom blinds in the morning -- you can take my word for it, and visit: The neighborhood is truly more diverse and interesting than it used to be, even at night, and especially in terms of places to stop and have a drink (or a dinner, a conversation, a cup of java, an out-of-body experience or whatever it is you have when you're not home).

It's just that you may get back home relatively early -- unless you come down here for some special event, like the generally uplifting, indoor-outdoor New Year's Eve party put on last year by the Post Office Pavilion (and planned as an annual event, happily), or the Halloween parties the Pavilion co- sponsored with WHUR and DC-101 last week, for example. Or for a concert at the Warner. Or for "Cats" at the National -- followed, as opposed to preceded, by dinner. (Washingtonians still tend to eat Before instead of After -- because it's a long, post-11 o'clock drive back to Bethesda or Fairfax, where a lot of "Washingtonians" live.)

These days, in any case, of all the restaurants and bars now within walking distance for theatergoers of all socio-geographic persuasions, the new Old Ebbitt Grill seems to be the most steadily crowded. Possibly this is due to its proximity to the National Press Building. Possibly -- because the food is generally decent, if not particularly inexpensive, at most anyplace down here -- this is a function of simple visibility: The Old Ebbitt is real easy to spot from a car. It is also the closest to the White House, with a brightly streetlamp-lit, tastefully spare exterior facing the Treasury building on 15th Street.

So get out of the car, go in and have a glass of house chablis ($2.50), and marvel at how they've managed to blend the old oddball character (i.e. mooseheads on the wall, Washington correspondents elsewhere) of the Ebbitt's former F Street quarters with the etched glass and gaslights of the new Metropolitan Square digs. Then, however, take a walk.

If the weather was warmer (and it was just a second ago, wasn't it?), you would definitely stop a block south at the Hotel Washington, take the elevator to "R" and have an obligatory vodka-grapefruit ($3.24, plus tip) at the Rooftop Terrace. From here you would see (to the south) most of lower Pennsylvania Avenue, the mall and Crystal City in the distance. To the east you would see Rosslyn and Georgetown in the distance, plus the White House in profile just a block away (and is that someone with a pizza box headed into the East Gate?). The hotel generally closes the terrace for the season in October, but, to quote the manager earlier this week, "if the weather keeps up like this I would like to stay open until Christmas."

Walk on, meantime, down to Pennsylvania Avenue itself, past the soon-to-be-born-again Willard Hotel, and over to the J.W. Marriott Hotel. Just after you enter from 14th Street, you ought to hear either a jazz trio or a pianist; this emanates from the Garden Terrace, the cocktail lounge directly off the Marriott's vast, chandeliered, peach-marble and art-deco-carpeted lobby. The chairs in the lounge are extremely comfortable, even if you're sitting on a wallet full of credit cards -- or especially so, actually.

At the far end of the Marriott ("That's right," says one well- dressed matron to another as they stroll through the lobby, "and you'll never see the Russians build anything like this . . . .") is the entrance to The Shops -- three extremely pretty levels of mostly, well, shops. Plus a couple of budding nightspots, including the Boston Seafood Company on the lobby level, with its raw bar and its regular bar (and a late happy hour, from 9 to 11) and the American Cafe downstairs.

The American Cafe is the more decoratively inviting, un-shopping-mall-like and better-located of the two (right beside The Shops' street entrance at 13th and F, with its trademark neon sign and ceiling accents visible from outdoors). Regular people -- meaning people not from Kansas City, not wearing blue blazers and ties and not likely to have long conversations about whether one's Porsche 924 should be driven without a bra (the black-net thing that covers the headlights, I think) -- seem to show up here semi-regularly, and stay slightly late, as if they lived nearby. The Cafe, unlike the shops of The Shops, which close at about 9, stays open until about midnight -- and some nights the wait for the after-theater crowd can seem discouragingly long.

Over on the other side of the National Theater, at J.J. MelBFlon's, they don't wait; the place closes at 9:30 -- which is a shame. Mellon's has a sophisticated, low-key Manhattanish feel, enhanced by lots of glass, subdued lighting and low-cut booths upstairs (there's a 200-seat dining room downstairs), a homemade pre-Elvis jukebox menu and a strangely unstuffy clientele that ranges from off-duty FBI to on-location movie producer.

Mellon's shares the tree-dotted (but poorly lit) brick terrace in front of 1201 Pennsylvania with the Bread Oven and its Artist's Room (live cabaret-type shows Thursday-Saturday nights; Dan Ruskin on the see-through grand piano otherwise), but the Bread Oven is more brightly lit than J.J. Mellon's -- and thus easier to see. Perhaps this helps explain why the Bread Oven is open 'til midnight most nights.

Across Pennsylvania Avenue, at the Post Office Pavilion, you'll find six restaurant-bars open until 1 a.m. amid the 50 shops and fast-food booths on two levels (most of which close at 9:30). These restaurant all have bars -- Richard's (seafood), Cafe Maxime, Blossom's (raw bar, white wine on tap), Hunan at the Pavilion, Fitch, Fox and Brown and FetBFtucine's -- but are best called nightspots mostly on Friday nights, when the after-work crowd stays late and the after-dark adventurers drift in more than sporadically. Otherwise, business slows considerably after 7 (quitting time for the frequently impressive -- and free -- musical acts booked on the central atrium stage by Pavilion management for lunchtime and after- work programs). The Pavilion is considering bringing in bigger- name acts regularly to the central stage -- and last Sunday night's Sun Ra date, a one-time benefit for District Curators and WPFW-FM, should offer much encouragement. AN AFTER-DARK SAMPLER ALONG THE AVENUE

AMERICAN CAFE -- 1301 Pennsylvania Avenue NW (13th and F). Open Monday 7:30 a.m. to 10 p.m., Tuesday to Friday 7:30 a.m. to midnight, Saturday to Sunday 11 a.m. to midnight. 737-5153.

BOSTON SEAFOOD COMPANY -- 1301 Pennsylvania Avenue NW. Open 7 a.m. to midnight Tuesday-Friday, 11:30 to midnight Saturday, noon to 10p.m. Sunday, 7 a.m. to 10 p.m. Monday. Two-for-one happy hour Monday-Friday 9-11 p.m. 737-9211.

BREAD OVEN ARTIST'S ROOM -- 1201 Pennsylvania Avenue NW. Open 7:30 a.m. to about midnight, Monday to Friday, 11 a.m. to 1 a.m. Saturday, closed Sunday. Piano at 8 p.m. Monday to Wednesday, show 8:30 Thursday to Saturday. 737-7772.

GARDEN TERRACE -- at the J.W. Marriott, 1331 Pennsylvania Avenue NW. Open 11 to 12:30 weekdays, noon to 1:30 weekends, live music Monday to Saturday nights. 393-2000.

J.J. MELLON'S -- 1201 Pennsylvania Avenue NW. Open 11:30 a.m. to 9:30 p.m. Monday to Friday, closed weekends. 737-5700.

POST OFFICE PAVILION -- 1100 Pennsylvania Avenue NW. Restaurants (not including the walk-up concessions downstairs) open 11 a.m. to about 1 a.m. Monday to Saturday, until 6 p.m. Sunday. For Pavilion events and lunchtime/afternoon concert information: 289-4224 during business hours.

ROOFTOP TERRACE -- at the Hotel Washington, 515 15th Street NW. Open 4 p.m. to 1 a.m. daily. 638-5900.