The Boucher micromethod was used to determine renin activity in the frontoparietal cortex, brain stem, hypothalmus, hypophysis and epiphysis of rat tissue in different functional conditions. In rats previously treated with an i.v. injection of 1-1.5 ml 10% NaCl, an increase of renin-like activity was found in the cerebral cortex, brain stem and hypothalamus; in the glandular tissue there was a significant decrease. In the paradoxical sleep-deprived rats, cerebral renin activity also increased. In contrast to the effect of hyperosmosis, darkness did not modify the renin-like activity in the pineal gland. The results are discussed in terms of the possible participation of the cerebral renin-angiotensin system in the autoregulation of the hydro-electrolyte balance.