The effects of acute volume expansion and acute salt loading on the plasma levels of immunoreactive atrial natriuretic peptides (ir-ANP's) were investigated in conscious adult (16 weeks) Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). Basal plasma concentrations of ir-ANP's were similar (108 +/- 9 pg/ml and 105 +/- 8 pg/ml, respectively) in hypertensive and normotensive rats. In both strains mean arterial pressure, heart rate and plasma catecholamines were unaltered by acute volume expansion but significantly elevated by acute salt loading. However, both acute volume expansion and acute salt loading increased plasma concentrations of ir-ANP's similarily in SHR and WKY rats, independent of blood pressure or plasma catecholamine levels. Our findings show that chronic hypertension in SHR does not result in changes in basal or VOL- and SAL-stimulated levels of circulating ir-ANP's.