Growth hormone and prolactin are secreted episodically in man and experimental animals. To investigate physiologic mechanisms of GH and PRL secretion, a series of experiments were performed in individual, unanaesthetized male and female rats. GH secretion in the male rat is characterized by intermittent surges that occur approximately every 3 h and are entrained to the light-dark cycle. Peaks reach 200--400 ng/ml and troughs are unmeasurable. PRL is secreted in more frequent episodes with a pattern distinct from GH. In the female rat, GH surges occur more frequently--approximately once each hour. PRL levels are low (less than 15 ng/ml) except on the afternoon of pro-oestrous when they surge to levels of 100--300 ng/ml. Prolactin rises 4--6 h before delivery. Levels decline rapidly at the onset of parturition and surge with each episode of suckling in the post-partum period. Growth hormone and corticosterone rise during delivery and remain elevated for several hours after delivery. Reinstitution of suckling after removal of pups causes an immediate rise in PRL and GH. The PRL response is sustained for 3--4 h, whereas the GH response is brief with return to baseline within 1 h. The time courses of the two responses are clearly independent. Stress in the male rat causes a rapid rise in PRL and suppression in GH. The PRL surge to stress is brief with return to baseline by 1 h. GH pulses are suppressed for up to 5 h after stress. These studies indicate that separate neuroendocrine control mechanisms exist for regulation of the episodic release of GH and PRL in the rat.