Brain activity during the spontaneous ovulatory surge of luteinizing hormone (LH) has been studied by measuring local cerebral glucose utilization (LCGU) by the [14C]-2-deoxyglucose method. The LCGU was determined in 37 brain areas and the pituitary gland in conscious, freely moving female rats in the morning and the late afternoon of proestrus. No increases in LCGU were detected, but, unexpectedly, there was a significant decrease in the LCGU measured in the afternoon compared with the morning of proestrus in the medial preoptic and anterior hypothalamic areas, the arcuate nucleus, median eminence and amygdala. Significant reductions in LCGU also occurred in the midbrain central grey and reticular formation. These results suggest that the LH and/or the prolactin surge is associated with a significant reduction in the activity of brain areas known to be essential components of the central control of gonadotropin and prolactin secretion. In the case of the arcuate nucleus and median eminence, for example, the results could be explained by a decreased activity of the opioid and dopaminergic neurons which are known to inhibit the release of luteinizing hormone releasing hormone (LHRH). Disinhibition of LHRH neurons would result in the increased release of LHRH into the hypophysial portal vessels. Reduction in the activity of the arcuate dopamine neurons could also play a major role in the prolactin surge. The decreased LCGU of the midbrain central grey may be related to the onset of lordosis behaviour which appears to be time-locked to the LH surge.