TAKE the MTA one stop past Park to Washington Station. Let the fat ladies with shopping bags sweep you up the rickety wooden escalator. Turn left at the tattered John Havlicek for hairspray, climb up past tattered John hawking Record Americans, and you will find yourself on Washington Street--address of Boston's best skin flick parlors.

Washington Street starts at the mouth of the Charles, sweeps southwest past Government Center, Z-slashes through the heart of West Roxbury, and plunges dead south through the suburbs of Dedham and Westwood. For most of these 25 miles, the street is dark and quiet.

Standing at Washington Station, on the eight-block strip from the Old South Meeting House to Michael Breen Square, you will see nothing but a jumble of neon and a jungle of blaring discotheques and smutty bookshops.

At night, policemen are posted at every intersection on this strip. A navy blue M.P. paddywagon watches over one of the discotheques. Gangs of well-liquored sailors and well-lacquered women stroll the street, stopping to chat in convenient alleyways. At Breen Square, a plump Polish girl will offer to show you her apartment.

TEN motion picture theatres line these eight blocks, including three skin flick parlors named, in patriotic Bostonian fashion, the Mayflower, the Pilgrim, and the State.

Posters outside these three theatres make good reading even for weekend windowshoppers. There are glossy stills from the films--mostly nude women with their breasts and bottoms blacked out with magic marker. This proves to be somewhat misleading, since in the picture these same parts remain covered with panties and pasties, but most regular viewers eventually catch on to the trick.

If you see the films, you may succumb to sneaking suspicions that some of the advertised shots never appear. This may be deliberately false advertising, but more than likely the scenes in question--certain to be the most sordid in the films--fell beneath a censor's scissors.

The three theatres marquees, with letters as large as any north of Times Square, often bear such inspired titles as "Lotita," "Fanny Mill," "Hawaiian Thigh" and "My Bare Lady."

This week, the Pilgrim features "Agony of Love" and "Wonderful World of Girls"--for "broad minded adults with young ideas." A gaudy poster for the first film proclaims it has been "banned over half the world!" If the other half should seek revenge, a familiar yellow and black sign hung just overhead offers some solace: "Fallout Shelter, Capacity 1145."

In the sidewalk case at the May-flower, a tawdry blonde beckons by-passers to "Infidelity Any Style" with "Come in Darling, my husbands (sic) at work." The film promises to be "Daring! Bold! A slice of life!"--apparently a new variation on the still more popular "piece of ass."

By the time you reach "Party at Lil's Place" at the State, the supply of adjectives (but not exclamation points) has apparently been exhausted. After such feeble attempts as "a scathing film of a girl without morals!" the poster-painter throws up his brush with "Her desire was always there!"

All three theatres charge $1.75 for admission--fifty cents more than the straight establishments on the street. If you measure flesh by the acre, however, this is the best buy since the Louisiana Purchase.

Theoretically "adults only" can get in on the deal, as the theatres announce in signs large and numerous. The Pilgrim, classiest of the three (occasionally showing films like Loves of a Blonde), asks patrons to please be 21 or older. The Mayflower and the State will have you if you're eighteen.

THE ticket-seller is a kindly old lady who looks up from a Screen Romances to demand proof-of-age. If you hand her a hastily marked-over expired driver's license, and act incredulous when she questions it, she will take your two dollars and wave you on in.

Just inside the lobby is the inevitable snack bar, but it gets little use, and the popcorn man doubles as ticket-taker. Since there are no inter-missions, and the coming attractions are the real show-stoppers, a trip to the snack bar or men's room means you miss a mile or more of skin. Two-bit milk taffy suckers sell the best.