Following complete or nearly complete hypophysectomy in dogs with thoracic inferior vena cava constriction, a striking increase in sodium (Na) excretion occurred concomitant with a fall in femoral venous pressure. The previously high venous pressure and renal Na retention were restored by administration of thyrotropin (TSH), growth hormone injection or further constriction of the thoracic inferior vena cava, but adrenocorticotropic hormone (ACTH) was without effect on these functions. The data indicate that pituitary hormones influence the accumulation of ascitic fluid by contributing to the maintenance of an adequate level of venous hypertension.