Three enzymes selected as representative of major metabolic pathways (malic dehydrogenase, of the citric acid cycle, lactic dehydrogenase, of glycolysis and glucose-6-phosphate dehydrogenase, of the pentose pathway) were measured by quantitative histochemical methods in individual hypothalamic nuclei during the 5-day estrous cycle of adult rats. Malic dehydrogenase increases significantly from low proestrous levels to a peak at estrus and then declines during diestrus in the following nuclei and areas of the anterior hypothalamus: medial and lateral preoptic, suprachiasmatic, supraoptic, and anterior. Significant peaks of lactic dehydrogenase occur more often during diestrus-3 in hypothalamic nuclei of the middle and posterior hypothalamus. Glucose-6-ρhosphate dehydrogenase has a biphasic pattern with peaks usually occurring during the diestrous period.