The effects of lysine-vasopressin (VP) and pepsanurin (PU) on the diuretic-saluretic action of ANF were investigated. Anaesthetized female rats under constant intravenous perfusion with isotonic glucose solution (0.6 ml/h/100 g body weight) were used. Blood pressure was recorded continuously and in the urine collected every 20 min, volume, NA and K excretion were measured. In each rat two intravenous boluses of either 2.5 ug synthetic rat atriopeptin II or an equivalent amount of a semipurified rat atrial extract were assayed. Thirty min before the ANF second bolus, an i.v. injection of lysine-vasopressin was given. Doses of 0.1, 1, 5, 10 and 50 mU were used. PU in the dose of 0.5 ml, obtained from 20 ml of human plasma was administered intraperitoneally, 40-60 min before the second bolus of ANF. The smaller doses of VP did not inhibit the urinary response to ANF, and the higher one (50 mU) produced a significant facilitation of ANF effect. Contrariwise, PU produced a considerable inhibition on water, NA and K excretion promoted by ANF.