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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.