We followed atrial natriuretic peptide (ANP) levels in 12 patients during anticoagulant treatment of pulmonary embolism. In each patient with this disorder ANP decreased during treatment. Concomitantly, plasma renin activity (PRA) markedly rose. The decrease in ANP was significantly related to the rise in PRA. No consistent changes in ANP or in PRA were observed in five patients with peripheral venous thrombosis receiving the same anticoagulant treatment. Plasma aldosterone levels did not change in either group.