SUMMARY Spayed female hamsters were given a 150 μg pellet of oestradiol s.c. (day 0). After s.c. placement of a 200 μg pellet of progesterone 7 days later, all animals displayed oestrous behaviour. Treatment with 200 μg progesterone suspended in propylene glycol or sesame oil, on days 4, 5 or 6 also induced an immediate period of heat but subsequent placement of a 200 μg pellet of progesterone on day 7 did not always result in oestrous behaviour. The animals did not show the lordosis response after placement of the progesterone pellet on day 7 when 200 μg progesterone in sesame oil had been given on either day 5 or 6. In contrast, when propylene glycol was employed as the vehicle, only injection on day 6 blocked oestrous behaviour on day 7. When the same amount of progesterone in propylene glycol was administered on day 5, as injections of 50 μg each over 12 h or more, progesterone in propylene glycol was more effective in blocking oestrous behaviour on day 7. The injection on day 5 of 100 μg progesterone in propylene glycol as four injections of 25 μg over 12 h did not block oestrous behaviour on day 7. The injection of 50 μg progesterone in propylene glycol on four successive nights induced a period of sexual receptivity after each injection. These observations indicate that the duration of the insensitive condition, developing as a result of a period of progesterone-facilitated heat, is a function of the amount of hormone causing the initial receptive period, as well as the duration of progesterone stimulation.