Continuous intravenous infusion of rat atrial natriuretic peptide (rANP) was carried out for 60 min in urethan-anesthetized rats. Plasma rANP (PANP) levels during 0, 12.5, and 50 ng/min rANP infusion reached 20.9 +/- 3.4, 61.2 +/- 12.3, and 228 +/- 30.6 pg/ml, respectively. Urinary sodium excretion (UNaV) remained unchanged during the 0 and 12.5 ng/min rANP infusion. In contrast, the 50 ng/min rANP infusion resulted in a 5.6-fold increase in UNaV without significant changes in plasma aldosterone levels and glomerular filtration rate. Isotonic saline infusion (116 microliter/min for 60 or 120 min) caused a significant increase in UNaV. UNaV in an experimental model of the syndrome of inappropriate antidiuretic hormone secretion (SIADH) was 5.0 times greater than in control animals. PANP levels in 60- or 120-min saline-infused and SIADH rats were 46.3 +/- 6.7, 39.0 +/- 9.0, and 35.6 +/- 3.2 pg/ml, respectively. These values were significantly higher than control values but failed to achieve a level at which natriuresis occurred during the infusion of rANP. These results suggest that endogenous ANP may not play a critical role in the induction of acute natriuresis in volume-expanded states, although it could be partially involved in such a natriuretic response.