The in vitro production of corticosteroids by adrenal slices from representative species of prosimians, Old World, and New World monkeys has been compared. The increase in blue tetrazolium-positive steroid production in response to ACTH was greatest in slices from New World monkeys and least in slices from prosimians. The F to B ratio, the relative production of cortisol (F) and corticosterone (B), was also highest for the New World monkeys and lowest for the prosimians. These data support the hypothesis that the F to B ratio is genetically determined. It is proposed that this ratio can be maintained within certain limits by each group by the activation or repression of adrenal 17α-hydroxylase. © 1969.