Normative values were obtained for triiodothyronine and thyroxine from four species of Old World primate (chimpanzees, rhesus monkeys, African green monkeys and talopoin monkeys) and a single species of New World primate (squirrel monkeys) represented by two subspecies, Colombian and Bolivian. The Bolivian squirrel monkeys exhibited the lowest values for both triiodothyronine and thyroxine. Male talapoins had the highest levels of thyroxine. Significant differences were found in levels of triiodothyronine and thyroxine between males and females of the same species and between the two subspecies of squirrel monkeys. Triiodothyronine:thryroxine ratios were consistently lower in the males of all species examined.