Previous studies suggested that exogenous atrial natriuretic factor (ANF) altered extracellular fluid partition between plasma and interstitium. The effect of ANF and a similarly effective diuretic dose of furosemide on whole-body transcapillary fluid movement was studied in anesthetized rats by measuring changes in urine and plasma volumes, while accounting for the volume of fluid administered. After 75 min of infusion, urine volume was 9.9 +/- 1.0 ml/kg in rats receiving ANF (0.5 micrograms X min-1 X kg-1), and plasma volume decreased by 3.7 ml/kg (P less than 0.01) compared with rats receiving vehicle only. In the furosemide rats, urine volume was 12.3 +/- 1.9 ml/kg, yet plasma volume was not significantly different from that in the vehicle rats. These results suggested that unlike furosemide, which allowed sufficient capillary absorption to maintain plasma volume constant despite the large diuresis, ANF attenuated whole-body net capillary absorption. The effect of ANF on capillary absorption was quantitated in anephric rats by measuring the increase in plasma volume after intravenous administration of hyperoncotic albumin. After accounting for the volume administered, whole-body net capillary absorption was less (P less than 0.05) in the rats receiving ANF (4.8 +/- 0.4 ml/kg) than in those receiving vehicle (5.9 +/- 0.2 ml/kg). Plasma protein concentration was greater in other anephric rats receiving a similar ANF infusion than in those receiving vehicle. These findings suggest that ANF decreased capillary absorption because of increased average capillary hydrostatic pressure and not because of altered capillary permeability.(ABSTRACT TRUNCATED AT 250 WORDS)