Male and female sexual behaviour was studied quantitatively by observing pairs of oppositely sexed, adult rhesus monkeys during regular hourly test sessions over 2 yr.; at all other times the animals were housed singly. Rhythmic fluctuations in the sexual invitations of both males and females, and in the refusal reactions of females, occurred in relation to the menstrual cycle. A decline in male mounting activity occurred during the luteal phase of the cycle, and resulted from two principal types of change in male—female interaction: in one, males stopped making mounting attempts, although females continued inviting (loss of female attractiveness), and in the other, males continued to attempt to mount, but females stopped inviting and began refusing (loss of female receptivity). Bilateral ovariectomy of females abolished all rhythmic variations, and the behavioural interactions were reduced to low levels; these were restored by subcutaneous injections of oestradiol into the females. These findings indicate that sexual invitations in a female primate are mediated by ovarian hormones, and that the changes in male—female interaction during the menstrual cycle can be understood in terms of endocrine-dependent changes in sexual invitations and their outcome.