Differently directed changes in metabolic activity of anterior hypothalamic nuclei's neurons in rats during hyperthermia, fever, and hypothermia were revealed with histochemical methods. During hyperthermia, the activity of energy metabolism enzymes increased as well as RNA content in the neurons of supraoptic, paraventricular and median preoptic anterior hypothalamic nuclei. This is shown by an increase in the metabolic activity of neurons of these nuclei. Metabolic activity in neurons of median preoptic nuclei decreased and was not changed considerably in neurons of supraoptic and paraventricular nuclei during endotoxin-induced fever. The development of hypothermia was characterised by a decrease in metabolic activity of neurons of supraoptic, paraventricular and medium preoptic nuclei. It is supposed that differently directed metabolic activity changes in neurons of anterior hypothalamic nuclei during hyperthermia are connected with the mechanisms of body temperature regulation (median preoptic nuclei) and neurosecretory processes (supraoptic and paraventricular nuclei).