THE GREENSTONE jar once stank of blood.

It now sits almost prettily in a case of plastic in "Art of Aztec Mexico: Treasures of Tenochtitlan" at the National Gallery of Art. Its rim is wreathed with 18 human hearts. The Earth Lord, Tlaltecuhtli, engraved on its base, wears skulls at knee and elbow. His hands and feet are clawed. His tongue is an obsidian sacrificial knife.

They laid her down upon the offering stone. They stretched her out upon her back. They laid hold of her; they pulled and stretched out her arms and legs, bending up her breast greatly, bending down her back, and stretching down her head taut, toward the earth. And they bore down upon her neck with the tightly pressed snout of a sword fish, barbed, spiny; spined on either side.

And the slayer stood there; he stood up. Thereupon he cut open her breast.

And when he opened her breast, the blood gushed up high; it welled up far as it poured forth, as it boiled up.

And when this was done, then he raised her heart as an offering to the god and placed it in the green jar, which was called the greenstone jar. --The Dresden Codex

Such greenstone jars were common once. More common were such deaths.

In the plaza of Xocotlan, the Spaniard Bernal Di'az, who fought beside Corte's, saw piles of human skulls so regularly arranged that one could count them, and I estimated them at more than 100,000. I repeat again there were more than 100,000. Andre's de Ta'pia was comparably astonished by the great skull rack at Tenochititlan, the Great Temple of the Aztecs: The poles . . . were crowded with cross sticks from top to bottom, and on each cross stick there were five skulls impaled through the temples: and the writer and a certain Gonzalo de Umbri'a counted the cross sticks and multiplying by five heads per cross stick from pole to pole, as I said, we found there were 136,000 heads.

Washington has witnessed other awesome exhibitions, but none as frightening as this one, as gruesome or as dark. The greenstone jar still horrifies. So does almost every object in this death-filled, blood-soaked show.

Mankind has not changed much and has not abandoned carnage in the 462 years since the Aztec empire fell. The Aztecs did not put the feet of innocents in fire to discover the whereabouts of gold, or tear bodies limb from limb on racks, or strangle disbelievers as the Christian Spaniards did. They dropped no atom bombs. We must believe the Aztecs sang, struggled and put up with their poverty as do their descendants in Mexico today. They were human beings like others, embedded in their culture, loyal to their customs, obedient to their gods.

But their piety still shocks.

Aztec gods demanded death. The rising of the sun, the continuing existence of the universe itself, depended on their being fed human blood and flesh. In 1487, at the Great Temple's dedication, the victims stood in four long rows, and each row stretched for miles. Perhaps 20,000--and perhaps 80,000--were sacrificed in just four days. The work was hard, the sun was hot.

And what happened to those corpses?

Eyewitness accounts, among those written down by Bernardino de Sahagu'n (1499-1590), who spoke the Aztec language, leave little room for doubt:

Having torn their hearts from them . . . they started the body rolling down the pyramid steps. It came to rest upon a small square below. There some old men, whom they called Quaquacuiltin, laid hold of it and carried it to their tribal temple, where they dismembered it and divided it up in order to eat it.

He makes the point repeatedly: . . . and they carried the bodies to the houses which they called Calpulli, where they divided them up in order to eat them . . .

They ate them in a sort of stew, with chilies and tomatoes.

We know they thought such dining a service to the sacred, a eucharist of sorts. They were not the only people of pre-Columbian America to feed on human flesh. The Tupinamba of Brazil also ate their prisoners. So did the Huron tribes of Canada. But the Aztecs raised the practice to unprecedented heights.

Certain modern scholars--Michael Harner of the New School, Marvin Harris of Columbia--argue that Aztec cannibalism was caused less by faith than by hunger. The Aztecs killed for meat.

"Traditional explanations of the vast scale of this slaughter," Harris writes, "depict the Aztecs as people obsessed with the idea that their gods needed to drink human blood who piously proceeded, therefore, to wage warfare in order to fulfill their sacred duty . . . But holy wars among states are a dime a dozen. The Jews, the Christians, the Moslems, the Hindus, the Greeks, the Egyptians, the Chinese, the Romans--all went to war to please their gods or carry out god's will. Only the Aztecs felt it was saintly to go to war in order to supply vast numbers of human sacrifices . . . So intent were the Aztecs on bringing back prisoners to be sacrificed that they would frequently refrain from pressing a military advantage for fear that they would kill too many enemy troops . . . This tactic cost them dearly in their engagements with Corte's who, from the Aztec point of view, seemed to be irrationally intent upon killing everyone in sight . . ."

The land the Aztecs worked was poor, its population large. Between A.D. 200 and 1200, writes Harris, the population of the Valley of Mexico increased "from a few tens of thousands to 2 million." The Aztecs had no cattle. Their diet, largely maize and beans, provided little fat. Often they were forced to eat insect eggs and algae scraped from stagnant ponds.

"Aztec cannibalism," Harris writes, "was not a perfunctory tasting of ceremonial tidbits. All edible parts were used in a manner strictly comparable to the consumption of the flesh of domesticated animals. The Aztec priests can legitimately be described as ritual slaughterers in a state-sponsored system geared to the production and redistribution of substantial amounts of animal protein in the form of human flesh."

Not everyone accepts his view. But in some deeply scary way Aztec art supports it.

Its quality is high, its power is amazing. The goldsmiths of the Aztecs knew the tricks of lost-wax casting. Their sculptors had no metal tools, yet managed to work basalt, porphyry and jadeite with extraordinary skill. Henry Moore thinks their stone work is "unsurpassed." They made life-size terra cottas. (One, unearthed a year ago--a fearsome "eagle warrior" preparing to take flight--is included in this show.) Aztec art is just as grand, just as strong and subtle, as that of the Assyrians, the Romans, the Egyptians. But in spirit it is different.

No art of other cultures is so consistently horrific or so void of hints of kindness.

King Tutankhamen's treasures caused us to imagine the softness of the Pharaoh's clothes, the genius of his cooks. No flowers sweeten Aztec art. It exists to evoke fear. Its faces do not smile. In memory this show becomes a concentrated writhing of claws, fangs, bones and talons, skulls and rotting monsters. The smell of death is everywhere. Nothing reassures.

The cylindrical receptacle--big as a hotel punch bowl--that is set into the back of the colossal carved-stone jaguar held still-warm hearts and blood. The knife-nosed bat that flies on "The Stone of the Death Monsters" clutches "smoking hearts." The downward-flying round-eyed owl inscribed on the same object pierces human livers with its talons.

That bird is no relation to Walt Disney's wise old owl. The Aztecs saw the owl, the catalogue informs us, as "a creature of ill omen, whose appearance and nocturnal calls were invariably believed to presage misfortune and death." The animals in Aztec art--the eagles, jaguars, scorpions, crocodiles, toads--are not friends of man. Most of them are killers. The wondrous snakes on view are rattlesnakes or coral snakes. The colossal toad displayed is poisonous as well. That grasshopper suggests a plague. Even the small rabbits on the "Box of Hackmack" snarl and show large fangs.

Though the catalogue contends "the characterization of Aztec art as an art of the macabre has often been exaggerated," the majority of objects here are suffused with death.

One large carved stone sculpture here represents a Cihuateotl, a spirit of a woman who, having died in childbirth, haunted Aztec crossroads intent on snatching children. Her nose has rotted off, her finger-claws drip blood. Two heads of Xipe Totec, the catalogue notes rightly, represent "a deity whose cult was one of the most macabre and horrifying ever recorded in the history of religion . . . This deity was propitiated with the sacrifice of numerous war captives and slaves. Dispatched by the usual heart extraction method, the lifeless bodies of the victims were then flayed. These human skins, including the faces, were worn by ritual penitents for periods of up to 20 days."

Even Aztec jewelry often tries to scare. One cast gold and turquoise necklace--borrowed from Harvard's Research Center for Pre-Columbian Studies at Dumbarton Oaks, the Washington institution that helped organize the show--bears 18 little skulls. Those skulls were cast in pieces so that when the wearer moved, the death's heads wagged their jaws.

Scholarship, in recent years, has significantly clarified Aztec iconography. The phallic rattle of the rattlesnake, and the way its rounded scales mimic grains of corn, its hugging of the earth and the way it sheds its skin, all associate the reptile with agricultural fertility. Yet like almost every force portrayed in Aztec art, those hissing, coiled snakes still feel malign.

No art that reflects life can fail to acknowledge pain, decay, mortality. Goya painted death, so did Caravaggio. Horror movies prosper. And much German painting, and New York painting, too, celebrates the monstrous. But the darkest Western art--because it is too passionate, angry, self-indulgent, dreamy or amusing--pales in comparison with the patiently smoothed objects in this deadly serious show.

The Aztec empire fell on a summer afternoon in August 1521. Its conquerors were ruthless. Aztec priests were strangled, Aztec temples razed. The Spaniards faithfully broke every statue they could find. Aztec gold and silver they quickly melted down. A few Aztec objects have managed to survive in European state collections--Dr. John Dee of England, Queen Elizabeth's court astrologer, did his divinations on an Aztec "magic mirror"--but almost all late Aztec art was intentionally destroyed. The thought of all that melted gold, of all those shattered statues, intensifies the sense of lost, of needless pain, that haunts the present show.

"Scratch a Mexican," says Eduardo Matos Moctezuma, the archeologist who supervised the Great Temple dig, "and you will find an Aztec."

Death still plays a role in Mexican popular culture. But the countless skulls that grin each November on All Souls' Day, and the skeletons cavorting in the 19th-century prints of Manilla and Posada, contain a kind of happiness. This show suggests no joy.

It's been thoughtfully installed--by Gaillard Ravenel and Mark Leithauser of the National Gallery staff--in a flowing, open space whose walls of earthy, reddish brown suggest the mud of excavations and the colors of dried blood. Mexican museums were generous with loans, as were art collections both here and abroad. H.B. Nicholson of UCLA and Elizabeth Boone of Dumbarton Oaks picked the objects shown. GTE Corp. helped pay the bills. "Art of Aztec Mexico" closes Jan. 8.