Plasma concentrations of immunoreactive atrial natriuretic peptide (ANP) was measured in spontaneously hypertensive rats (SHR) during chronic salt loading (1.5% NaCl in drinking water). During the 3-week experimental period mean arterial blood pressure, heart rate, urinary sodium excretion and body weight was assessed in salt-loaded as well as in control rats. The sodium excretion was more than 10-fold increased in the rats on the high salt diet. The plasma ANP concentration was significantly increased only 24 h after the start of the high salt intake. Thereafter plasma ANP concentrations were not significantly different from values obtained in control rats. The blood pressure was significantly increased after 3 weeks on the high salt diet. At the end of the 3-week experimental period the rats were subjected to a 10 and 20% acute volume expansion with homologous whole blood. During this intervention the increase in plasma ANP concentrations was blunted in the high salt rats compared to the control group. It is concluded that during chronic salt loading in SHR there is an initial rise in plasma ANP levels and that other hormonal and neuronal systems are more important in the long term maintenance of fluid and electrolyte balance.