BACK WHEN the Armed Forces Medical Museum was on the Mall, it ranked as a major attraction, right up there with the Smithsonian and the Washington Monument. As many as a million tourists a year visited the old red-brick building to marvel at the skeletons, preserved body parts and other curiosities.

But when the building was razed in 1968 to make way for the Hirshhorn Museum, the collection was moved uptown to the Walter Reed Army Medical Center, and the flow of tourists slowed to a relative trickle -- only about 35,000 last year.

That's a shame, because the museum, now marking its 125th anniversary, is definitely worth a visit.

Some of the more gruesome exhibits -- two-headed babies and so forth -- are no longer on view.

"We've tried to tone down the sensationalism," says programs officer Sheila Pinaker. "We're more interested in teaching visitors about the history of medicine and medical research. We especially encourage school groups. In the D.C. school system, for example, sixth-graders are taught about the human body. The material displayed here fits right in with the curriculum."

That isn't to say some exhibits don't appeal to morbid curiosity.

On a recent visit, Marie D'Angelo of Stamford, Connecticut, and her 12-year-old son Ted were riveted by an elephantiasis victim's massively enlarged leg, amputated in 1890 and preserved in a huge glass jar."

"Oh, is that gross or what? Yuck!" Mrs. D'Angelo mumbled to no one in particular, while her son opined that the leg would really impress the kids back home.

There's also a trichobezoar (giant hairball), removed from the stomach of a 12-year-old girl. Ted gives that the once-over, too, and thinks it's pretty swell.

On a less bizarre note, Gary Bennett, a physics student from Silver Spring, is making pencil sketches of some of the more than 200 microscopes on display (the museum's collection numbers more than 800 -- the largest in the world).

"Some of the early examples are amazing," says Bennett, "especially considering the state of technology at the time. These were precision instruments . . . works of art, too."

Some are, in fact, just that. An 18th-century example by Edward Scarlett of London is covered with red leather embossed with gold fleur-de-lis and mounted on a polished mahogany base with brass fittings.

A late-model scanning electron microscope is strictly business, though, with a switch- and light-studded console that would be at home aboard the space shuttle.

Also shown is a vast array of medical implements, ranging from early scalpels, skull trephines, autopsy kits and hypodermic syringes through modern plastic and stamped-metal throwaways.

Intriguing is a Civil War surgeon's kit, complete with a wicked-looking amputation saw and gleaming forceps for extracting bullets and shrapnel.

Photo displays outline military contributions to medical progress, including Army officer Walter Reed's discovery of the cause and prevention of yellow fever in the early part of the century. Other exhibits outline the armed forces' role in developing new surgical procedures and in combatting diseases such as typhoid fever, malaria and meningitis.

One fascinating presentation, using realistic models and actual photos, shows how Civil War era surgeons performed restorative surgery on hideously disfigured soldiers. The results, while not up to today's standards of plastic surgery, were nonetheless amazing.

The museum's collection of human organs is extensive, with preserved brains, hearts, livers, spleens, lungs and glands of every description on display. Normal examples are shown along with some that have been ravaged by diseases such as cancer and TB, or lacerated by knife, gunshot or explosion.

One entire case is devoted to tissue preservation and display. Modern techniques such as immersion in alcohol and formalin or impregnation with plastics are explained, while an ancient Peruvian mummy and a shrunken head prepared by Jivaro Indians testify to the effectiveness of using dry heat to preserve flesh.

An exhibit of the embalmed head and shoulders of a young girl who died in Kentucky in the 1850s is haunting. Her hair, eyelashes and lips are intact, her features remarkably like those of a farm girl one might find in modern Appalachia.

There are exhibits of major historical significance, too. The bullet that killed Abraham Lincoln is on view, as are bone fragments recovered from the skull of our 16th president. Also preserved is a portion of the spinal column of President James Garfield, showing the path of the bullet fired by disappointed office seeker Charles J. Guiteau.

One of the museum's longest-running and most popular exhibits is the leg bones of Maj. Gen. Daniel E. Sickles.

Sickles' right leg, shattered by a 12-pound cannon ball at Gettysburg, was amputated in the field. He had the severed limb packed off to Washington along with the projectile and a note bearing the inscription, "With the compliments of Major General E.E.S."

He survived the war and for many years visited the museum on the anniversary of the amputation.. ARMED FORCES MEDICAL MUSEUM

The museum of the Armed Forces Institute of Pathology is in Building 54, at 14th and Dahlia NW, on the grounds of Walter Reed Army Medical Center (main entrance at 6825 16th Street NW).

Admission is free. Hours are 9:30 to 4:30 weekdays, 11:30 to 4:30 weekends. Closed Thanksgiving, Christmas eve and day, and New Year's eve and day.

School and study groups are welcome; to arrange tours, call 576-2348.