The plasma levels of immunoreactive (IR)-ANF have been evaluated by radioimmunoassay in several models of experimental hypertension and in human hypertension. In all models of experimental hypertension so far studied, the plasma levels of IR-ANF are consistently increased. This is accompanied by a decrease, at certain time intervals, of the IR-ANF levels in the left atrium. In human essential hypertension, the plasma levels of IR-ANF are not increased except in the severe form (diastolic blood pressure above 110 mmHg). In renovascular hypertension, the peripheral levels of IR-ANF are not different from the normal levels but are increased above normal in aortic blood.