Already friends joke with Lafred Meyer about his first novel, calling it "a rare treat" or "a gutsy book." But in anthropological circles, the novel coauthorered by Meyer is no laughing matter. Its gruesome subject: cannibalism among the Aztecs in the 16th century, an issue hotly debated by social scientists.

"It was like the Kansas City of Middle America," Meyer, 42, says of the Aztec Empire. "It was a vast system for getting amino acids. Religious sacrifice was a rationalization, and pyramids were merely elaborate butcher blocks."

That theory was first suggested several years ago by Meyer's co-author, Dr. Michael Harner, an anthropology professor at New York's New School for Social Research. Not everyone was enthusiastic about Harner's findings, especially those Mexicans just rediscovering their Indian roots. A couple of years ago Harner discussed his research with Meyer.He's a former editor of Natural Science, the short-lived Science edition of The Saturday Review, and he currently is managing editor of Science 80, the new magazine the American Association for the Advancement of Science will begin this fall

"I said, "My God, this is incredible, it's an amazing comment on human nature,"" Meyer recalls. "The Aztecs had achieved a high degree of jurisprudence, astronomy, was a highly civilized society. It was probably the only society that practiced institutionalized and dietary cannibalism at the same time. In one festival, they butchered 75,000 folks."

Though Harner had published his conclusions in scientific journals, Meyer suggested using the theory as a basis for a novel. The result, Cannibal, will be published later this month by William Morrow & Co. Based partly on historical fact, the novel is the saga of a Spanish conqueror sent to Mexico to find a mysterious European who joined forces with the Aztecs to teach the Indians to fight plundering Europeans. Harner and Meyer hope to do for anthropology what author Michael Crichton or Robin Cook did for medicine: use fiction to bring ideas with scientific merit to a popular audience.

For many authors, writing sex scenes is the hardest part of fiction; for Meyer, the pages involving cannibalism were "emotionally hard to write." But a background in natural history helped steel him.

"Everyday on the way to work I used to go past a hall and see displays of the origin of the universe and creation of the earth," says Meyer. "I had a different perspective on human beings. I can easily see that when times are tight, all human beings, like Aztecs, will do what they need to survive. They're like any other mammal - they react to stress and the scarcity of food."

Given the grim nature of cannibalism, it's probably natural that Meyer uses humor to mitigate the subject's impact.

"At our publishing party," Meyer says, "I do intend to serve a stew of uncertain origin." CAPTION: Picture, no caption, By Joel Richardson