To test the hypothesis that ANF increases the efflux of fluid from capillaries we determined the effect of synthetic (1–28) ANF on hematocrit (Hct) and plasma volume (PV) in 24 hr bilaterally nephrectomized rats. Results were compared with those obtained in bilaterally nephrectomized rats receiving saline alone (S) or an infusion of sodium nitroprusside (NP) to produce a similar blood pressure lowering effect as ANF. PV was determined by the volume of distribution of RISA 10 minutes after its i.v. administration. After control periods (C) rats were infused for 40 minutes with saline alone (group A, n=8), (1–28) ANF, 0.3 μg.min⁻¹/kg body wt (group B, n=9), 1.0 μg.min⁻¹/kg body wt (group C, n=8) and NP, 1.0 to 2.0 μg.min⁻¹/kg body wt (group D, n=8). Recovery periods (R) were performed 45 minutes after the experimental periods (E). During E periods, mean arterial blood pressure did not change in group A and fell significantly (p < 0.01) in the other groups. Group B: C = 89±4; E = 82±4 mmHg. Group C: C = 106±7; E = 92±7 mmHg. Group D: C = 104±5; E = 90±5 mmHg. Neither saline or NP changed Hct, whereas, ANF significantly (p < 0.01) increased this parameter (Group B: C = 43.4±0.7; E = 46.3±0.6% and Group C: C = 44.5±1.2; E = 47.4±1.2%). ANF significantly decreased PV (3.86±0.14 ml/100 gm body wt) as compared to S (4.51±0.22 ml/100 gm body wt; p < 0.05) or NP (4.69±0.11 ml/100 gm body wt; p < 0.01). The average decrease in PV of 14% corresponded to the average increase in Hct of 7%. Results demonstrate that ANF increases the efflux of fluid from capillaries. This effect explains, at least in part, the ANF-induced decrease in cardiac output and blood pressure in normotensive animals and may play an important role in the homeostatic regulation of plasma volume.