A study was made of the effects of hypothalamic deafferentation on LH and prolactin levels in the systemic circulation of the unanesthetized, freely-moving rat. Blood was withdrawn continuously at the rate of 0.5 ml/h for 3–5 successive 2 h periods, and the samples were assayed for plasma LH and prolactin by radioimmunoassay. Between sampling periods, the resuspended red blood cells were returned to the donor, with sufficient heparin to prevent clotting. In intact individuals, the proestrous surges of release of both LH and prolactin were clearly demonstrated by this integrated collection technique. In completely deafferented rats, the plasma levels of both hormones remained low and constant with minimal fluctuations. The LH and prolactin levels in anterior-deafferented rats were higher and showed relatively large fluctuations, suggesting nervous control of both hormones via afferent inputs reaching the basal hypothalamus from lateral or posterior directions.