Acute hypophysectomy (a.h.) (i.e. pituitary ablation 120 min prior to the experiment) abolished the ability of anaesthetized rats to excrete sodium following isotonic saline load (4% body weight). The diuretic response remained even slightly increased as compared to control sham-operated animals (s.o.) (maximal UNaV a.h. rats +/- SE: 0.65 +/- 0.10 mumol/min vs maximal UNaV in s.o. rats: 4.54 +/- 0.50 mumol/min). Alpha h-atrial natriuretic factor 1-28 (ANF) in a doses of 3.3 micrograms/animal.40 min restored natriuresis in a.h. rats following isotonic saline load to the level observed in s.o. rats (4.25 +/- 0.64 mumol/min), the change being transient. A decrease of blood pressure to 78.6 +/- 3.21 mm Hg during ANF infusion was followed by a reduction in the natriuretic effect of ANF, while the diuretic effect remained unchanged. Based on own results and literary data it could be suggested that impairment of ANF release or its renal effect are involved in the abolishment of homeostatically effective natriuresis in a.h. animals.