In rat and bovine lenses, the primary source of intracellular glutamate has been shown to be glutamine transported from the surrounding fluids, whereas extracellular glutamate is less readily utilized. For comparison, glutamine and glutamate metabolism were studied in a primate. Fresh, intact lenses from Rhesus monkeys (Macaca mulatta) were incubated in balanced salt medium containing [15N]glutamate (Group A) or amino-labeled [15N]glutamine (Group B). In contrast to other species, the monkey lenses metabolized the glutamate more rapidly than the glutamine, although glutamine entered the lenses more rapidly than glutamate. Formation of labeled aspartate, alanine, proline, and serine was more rapid in Group A than in Group B, but labeling of the lenticular glutamate + glutamine pool was more rapid in Group B. This indicated that, in monkey lenses, deamidation of glutamine is sufficiently slow to limit the entry of glutamine into pathways requiring or preferring glutamate. The difference in rates of deamidation of glutamine in intact monkey lenses and rat lenses was confirmed by measuring the rate of ammonia release by lenses incubated with glutamine.