SUMMARY: Progesterone injected intramuscularly at 09.00 h on the day of prooestrus in the 4-day cycle of the rat decreased plasma luteinizing hormone (LH) concentration for 2 to 3 h, but by 5 h after injection had triggered an abrupt, premature, and sustained increase in LH secretion. Between 4 and 5 h after injection sufficient LH had been secreted to raise the plasma LH concentration to more than 20 ng/ml and to induce ovulation in the majority of rats. The facilitatory effect of progesterone on LH secretion could be prevented, or extinguished after it had begun, by the administration of sodium pentobarbitone. The effects observed after administration at the prooestrous stage of the cycle are compared with those seen after progesterone injection under other conditions.