These studies attempted to identify the factors which stimulate the postpartum preovulatory surge of gonadotropin secretion in the rat. The time of parturition on Day 23 of pregnancy slightly affected the time of the postpartum gonadotropin surge which occurred that afternoon. Animals delivering early in the morning had postpartum gonadotropin surges earlier in the afternoon than animals delivering later in the day. To ascertain whether an afternoon time of day signal stimulates the postpartum gonadotropin surge, pentobarbital was administered immediately postpartum. Pentobarbital blocked the postpartum gonadotropin surge on the day of parturition in only 6 of 17 animals. Surprisingly, these animals all had gonadotropin surges the following day despite a large continuous suckling stimulus. To determine whether the process of parturition stimulates the postpartum gonadotropin surge, pups were delivered by cesarean section on Day 22 of pregnancy. Luteinizing hormone (LH) surges were not detected in cesarean-sectioned animals on the day of the operation, nor on the following day. However, cervical stimulation in the morning resulted in LH surges in 5 of 9 cesarean-sectioned rats later that day, at a time coincident with the afternoon time of day signal. These studies suggest that the postpartum gonadotropin surge results from an afternoon time of day signal which is activated by the cervical stimulation of the labor process.