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Progesterone-filled constant-release implants facilitated the induction of sexual receptivity in ovariectomized rats given implants of oestradiol-17β precisely 32 h before testing, irrespective of the time of implantation. Inhibition by progesterone implants of the behavioural response to an injection of progesterone occurred after the facilitation 32 h after oestradiol implantation. Sexual receptivity could be induced in pseudopregnant rats in the absence of progesterone treatment by injection of 1 µg oestradiol 32 and 16 h before testing at a time when endogenous serum levels of oestradiol were low and progesterone levels were high. The behavioural response of ovariectomized rats implanted with oestradiol and tested daily was unaffected by implantation of progesterone at the time of oestradiol implantation, although serum levels of progesterone varied with the number of progesterone implants inserted. Inhibition by progesterone implants of the behavioural response to an injection of progesterone 6 h before behavioural testing occurred only if the progesterone implants were present for at least 32 h of a 48 h period. Serum levels of progesterone were raised within 1 h of progesterone implantation and declined within a 6 h period after implant removal. It is concluded that progesterone does not inhibit the behavioural effect of oestradiol and that progesterone does not play an inhibitory role in the regulation of the behavioural oestrous cycle in our strain of rats.