Pairs of adult rhesus monkeys of opposite sexes (12 pairs) were studied during 1-h mating tests (986 tests). Ovariectomized females received 5 microgram oestradiol intravaginally throughout to keep them attractive to males and maintain a relatively constant level of male sexual activity. Females were given subcutaneous silastic implants of testosterone to change their sexual receptivity. A threefold change in sexual invitations resulted from the implantation and removal of testosterone in the absence of any major changes in the behaviour of their male partners. These changes in invitational behaviour therefore reflected an action of the hormone upon an internal mechanism within the female subserving sexual motivation which was independent of changes in the behaviour of the male. Radioimmunoassay of plasma testosterone (109 samples) and plasma oestradiol (86 samples), and gas-liquid chromatography of vaginal secretions (306 samples), facilitated the monitoring of variables needed to interpret the results.