Twelve days after hypophysectomy depleted atrial natriuretic polypeptide (ANP) concentrations were measured in the plasma and in 8 of 18 microdissected brain nuclei of rats. Reduced ANP levels were found in brain structures (subfornical organ, organum vasculosum laminae terminalis, preoptic and hypothalamic periventricular nuclei, paraventricular nucleus, lateral hypothalamic area), which are directly involved in the central regulations of salt and fluid homeostasis, as well as in the medial amygdaloid nucleus and the locus ceruleus. ANP concentrations in the median eminence, medial preoptic and arcuate nuclei did not alter by hypophysectomy. Elevated ANP concentrations were measured only in the supraoptic nucleus of hypophysectomized rats.