WHEN evening falls on a summer Friday at the Marine Corps Barracks, the shadows of the hugh old maple trees stretch across the parade ground and present-day Washington has a date with the past.

While ragged young street kids from nearby housing projects zip past on skateboards, white-gloved women in cotton dresses and broad-brimmed summer hats step from their cars and Marines in dress blues, brass buttons gleaming, cluster at the brick and boxwood gateways in the gathering dusk.

From the garden behind the commandant's house, comes the sound of laughter and conversation and the tinkle of ice in glasses, and through the bushes flash white uniform and gold braid.

Inside the quardangle on the parade ground, the Marine Band in scarlet tunics plays Rossini. It is the hour of the Evening Parade, a chapter out of Kipling rewritten each week from mid-May to mid-September in a time-frozen city block beside the Southwest Freeway.

Outside the 177-year-old barracks walls lies a city of sirens and transistor radios, of T-shirts and cutoffs and all the urban tumult of the late 20th century. But inside on Friday evening waits an elegant, neatly ordered world of discipline and pomp that echoes of empire and sacrific and pride. Served up in a sharply honed ritual, at times outrageous in its theatricality, it remains nonetheless the most affecting pageant in the nation's capital and one of the best free shows of any kind around.

The Evening Parade is something of an anomaly in Washington: a 20-year tradition better known to most visiting tourists than to the people of the metropolitan area itself.

The 3,200 seats on the parade ground bleachers are usually reserved weeks before every performance, and tourists queue up more than an hour before parade time for a chance at replacing no-shows. But many on Capitol Hill, within earshot of the drums and bugles of both practice and parade, have never looked behind the barracks walls.

The brick buildings at 8th and I Streets SE comprise not only "the oldest post of the corps," but some of the oldest structures in the city of Washington. President Thomas Jefferson personally selected the barracks site in 1801 to garrison the Washington Navy Yard nearby. The white bay-fronted commandant's residence at the north end of the quadrangle ranks as the city's oldest public building in continuous use.

The 900 marines presently assigned to the barracks still guard the Navy Yard gate, as well as Camp David, the president's mountain retreat in Maryland and - in emergencies - the Capitol and the White House. They also take weapons, tactics and leadership training, and serve as cooks, clerks and administrators for Marine Headquarters in Washington, and for the Marine Corps Institute correspondence school. But their primary mission is ceremonial: serving as priests and guardians of the military rituals of state and reaffirming the values of discipline and duty. They serve as body bearers at Arlington Cemetery and honor guards at the Department of State. And Friday nights in the summer they honor the role of the guardian itself.

"Good evening, ma'am," says the cleft-chinned sergeant in the dress blues to the startled 10-year-old girl in the pink organdy dress waiting in line with her parents at Gate 6.

"Welcome to the Marine barracks. May I escort you in?"

He speaks without a hint of condescension or irony, and as the girl makes a rapid transition from alarm and confusion to positively regal enchantment, he offers his arm and leads her through the brick gates and boxwoods and into the past.

Inside the quadrangle in the soft summer twilight, the manicured parade ground glows a velvet green. And arranged on the stands to the south and west sits all of America: a Boy Scout troop from Huntington, N.Y., a retired admiral and a one-time draftee, a fat woman tourists in shorts with her hair in curlers and an off-duty soldier in fatigues, a young Japanese-American couple from San Francisco and a Wyoming family of four, a black mother from North Carolina in a new dress, proudly watching her officer son perform.

From the moment two spotlights converge on the ship's bell at the foot of the barracks flagpole and the time orderly signals "two bells," the Evening Parade is locked into a time-worn ritual - a ritual part Roman, part British, all Marine.

A bugle sounds and from the darkness at the south end of the parade ground comes drum thunder and silver bugles and Sousa. A bow-legged bulldog in Marine Corps tunic (Chesty V) struts out as the barracks mascot and sits in the spotlight on command. As he departs a fanfare sounds from above and another spotlight finds eight red-coated buglers tah-rahing on the barracks parapets.

Voices sound off one by one from the darkened arcade behind each of the spotlights. Only the band and drum and bugle corps are visible. And then, as the drums thunder anew and the bugles blare, musical units begin to move and from the darkness behind them the first marching units move on to the suddenly floodlit parade ground in white-hatted symmetry. On and one they come, six platoons with every shoulder-borne rifle in perfect line, every brass button gleaming. The band plays "Born Free" - as a march.

The parade is on - nearly an hour of precision marching and intricate maneuver, a tour de force of self-discipline and showmanship. One hundred-fifty steel-plated rifles swing from shoulder to ground in one white-gloved motion, their butts striking the concrete walkway with a single echoing thump. Platoons march back and forth through close ranks, barely missing each other with their fixed bayonets. A silent drill team executes a ten-minute kaleidoscope of maneuvers without oral command, culminating in a mock "inspection" in which team members standing rigidly at atttention twirl, toss and fling fully operational 11-pound M-1 rifles around like sticks and - in one case - exchange two simultaneously by catapulting them behind and over their backs. Officers salute the reviewing stand with saber signals dating from the time of the Crusades. And at the end, when the troops have passed in review beyond the Krupp cannon marines captured near Tientsin in the Boxer Rebellion, a spotlight finds a lone, scarlet-coated figure silhouetted high on the parapets playing taps.

The origins of the military parade are as old as man, traceable to the earliest tribal rituals of war and the hunt. The Evening Parade grew from such traditional military ceremonies as tattoo, retreat and lowering the colors which began in the 17th century when British troops were stationed in Holland.

To get roistering redcoats out of the local inn and back to the barracks at night, a drummer was sent through the streets to signal a "tap toe" - a Dutch expression for turning off the taps on a wine keg and ending the sale of drink. In time, to make certain the signal was heard, a fifer was added, and eventually the whole fife and drum corps paraded regularly at the end of the day in what had become know as a tattoo.

The retreat ceremony grew out of British military tradition in North America when early colonists grazed cattle and grew their crops outside the walls of fortified towns. At sunset each day a call was sounded on a horn to signal those outside the town to return within the safety of the gates. The flag was also lowered and the garrison mustered so the night guard could relieve the day guard.

On the manicured parade ground at 8th and I, the officers with their sabers and medals echo that age when the guardian role - like the citizen's obligation - seemed clearer cut, and for many in the stands, a visit to the Marine Barracks takes on a quality of a pilgrimage in time.

But the parade as we know it today probably owes most of the ancient Romans who devised the idea of keeping their troops in step. Although primitive armies "marched" bunched together in rough columns, it was only with the invention of tactical formations like the phalanx that precision movements of large units in close ranks became important. That produced marching in step to the cadence of the battlefield drum.But it also produced something else: the subordination of the individual warrior to the military unit as a whole. For the effectiveness of a military unit could now equal more than the sum of its individual members provided discipline was maintained.

The invention of the musket, which rewarded disciplined firing by rows, carried te concept still further, and troops drilled endlessly in precise, methodical weapon-handling to become a useful part of a purpose larger than themselves alone.

In a sense the Evening Parade commemorates all of that: the honor of duty to something larger than oneself, whether a drill team, a nation, or an ideal, and long after the parade is over, its message lingers, soaring over doubts and memories with the haunting clarity of the bugle call from the parapets at 8th and I.