SUMMARY The assays of testosterone and corticosteroids in plasma from adult male rhesus monkeys using competitive protein-binding and radioimmunoassay techniques are described. The radioimmunoassay for testosterone was conducted without chromatography and, therefore, additionally estimated 17β-hydroxy-5α-androstan-3-one (dihydrotestosterone). Levels of testosterone in the peripheral plasma of 14 intact male rhesus monkeys showed marked fluctuations over a period of 24 h. Concentrations of testosterone at 22.00 h (1776 ± 814 ( ± s.d.) ng/100 ml) were approximately double those at 08.00 h (858 ± 407 ng/100 ml), 12.00 h (898 ± 316 ng/100 ml) and 16.00 h (784 ± 530 ng/100 ml). Castration resulted in low plasma testosterone levels (85 ± 29 ng/100 ml), and the increases at 22.00 h were no longer observed. In intact males, the 'basal' plasma corticosteroidlevel(08.00 h) was 22·4 ± 6·0 μg/100 ml. Administration of synthetic corticotrophin raised plasma corticosteroid levels without changing plasma testosterone concentration. Because plasma testosterone levels were not related to changes in adrenocortical activity, the noctural rises appear to be due to changes in testicular secretion.

SUMMARY Sexual behaviour was studied quantitatively by observing six pairs of oppositely sexed, adult rhesus monkeys during 209 hourly test sessions conducted over a period of 7 months; at all other times, animals were housed singly. The effect on male—female sexual interaction was studied when ovariectomized, oestrogen-treated females were given s.c. injections of progesterone. Administration of 25 mg. progesterone resulted in a highly significant decrease in male mounting activity, in ejaculation, and in the number of sexual invitations made by females. At the same time there was a highly significant increase in the number of female refusal reactions. In three pairs, mounting attempts by males remained constant and the decline in the number of mounts resulted primarily from a large increase in refusal reactions. In three other pairs, the decline in the number of mounts resulted, primarily, from a marked decrease in male mounting attempts. The two principal mechanisms, loss of female receptivity and loss of female attractiveness, previously observed during the luteal phase of the female's menstrual cycle, were thus reproduced by giving progesterone to the ovariectomized, oestrogen-treated female partners.