After acute blood volume expansion (BVE) in the rat, diuresis and natriuresis are reported to be minimal in rats 20 to 30 d of age, but increase to mature levels by 40 d of age. To evaluate the role of atrial natriuretic peptide (ANP) and its renal action in BVE, anesthetized Sprague-Dawley rats were studied at 25 to 30 (group I) and 45 to 50 d of age (group II). Hematocrit, mean arterial pressure, glomerular filtration rate, urine flow rate, urine sodium excretion, urine cyclic GMP excretion, and plasma ANP concentration ([ANP]) were measured before and after infusion of donor littermate whole blood, 2.5% body wt (BVE), and in time controls (no BVE) in each group. Baseline hematocrit, mean arterial pressure, and glomerular filtration rate were greater in group II than group I, but urine flow rate, urine sodium excretion, urine cyclic GMP excretion, and [ANP] did not differ. BVE caused a prompt increase in urine flow rate, urine sodium excretion, and [ANP], but not urine cyclic GMP excretion, in both groups, but there was no difference in the response between groups. Additional groups of rats of the same ages as groups I and II studied using a protocol similar to that of a previous report also showed the “mature” diuretic and natriuretic response even in the younger animals. We conclude that there is no further maturation of the renal response to acute BVE in the euvolemic rat after 25 d of age. The increase in [ANP] after acute BVE in the immature weanling rat is consistent with a role for ANP in mediation of the renal response.