The amplitude of the electrical activity of the ventromedial hypothalamic nucleus (VMH) and lateral hypothalamic area (LH) was evaluated in rats after 24-hr food deprivation to determine the effects of the diet they had received before fasting. Bioelectrical potentials were registered by means of monopolar electrodes. In rats after 24-hr food deprivation an increase in the absolute values of the amplitude of the electrical activity of the VMH and LH was found; this effect was more pronounced in the LH. Comparing the amplitude of the electrical activity of the VMH and LH, we found it to be always higher in the VMH in ad libitum fed rats, while in 24-hr food-deprived rats the electrical activity was always higher in the LH. The extent of changes depended on the diet the rats had received before fasting--they were most marked in rats fed on a high-protein diet and less marked in rats fed on either a standard or a high-fat diet before food deprivation.