Experiments were carried out to investigate the hypothalamic control mechanism for prolactin (PRL) and luteinizing hormone (LH) secretion in the female rat. Anterior medial preoptic nucleus (AMPO) or suprachiasmatic nucleus (SCN) lesions were produced by passing 5–10 µA of direct current (tip negative). Persistent estrus (PE) began as early as 6 days and as late as 30 days after electrolytic lesioning. Blood samples obtained during diestrus, proestrus and estrus revealed well-described profiles of plasma PRL and LH in sham-lesioned animals, indicating that our cannulation and blood sampling procedure had no adverse effects on the plasma hormone levels. Individual sham-operated animals sampled on successive or alternate proestrous afternoons showed precise timing of the PRL and LH surges. However, when a shift occurred in the PRL surge a comparable shift would also occur in the LH surge, indicating a coupling between the mechanisms regulating the PRL and the LH surge. The AMPO-, SCN- or combine-lesioned PE animals exhibited low basal levels of plasma PRL and LH. Small secretory bursts occurred one to three times during the 6-hour sampling periods. Animals with incomplete SCN lesions had plasma PRL titers significantly higher than the other 3 groups. Plasma progesterone levels were significantly lower in the PE animals (p < 0.01), whereas plasma estrogen levels were not significantly different from proestrous controls. These experiments indicate that during the afternoon of proestrus, the surges of plasma PRLand LH are very precise in the time of onset. Moreover, the mechanisms controlling the surge of PRL and LH are temporally coupled. Lesions of the AMPO or the SCN cause very low levels of plasma PRL in the presence of elevated plasma estrogen levels, and accumulation of LH-releasing hormone in neurons throughout the preoptic-hypothalamic continuum. In conclusion, these data indicate that the AMPO and the SCN act as a unit to initiate the surge of PRL and LH.