Controversy persists on whether atrial natriuretic factor (ANF) raises renal solute excretion simply by increasing glomerular filtration rate (GFR) or whether it directly inhibits tubular transport (independent of changes in flow rate). Free-flow micropuncture techniques were used in 10 Munich-Wistar rats. ANF caused a significant increase in single-nephron and whole-kidney GFR (41.6 +/- 1.4 to 52.7 +/- 2.3 nl/min and 0.95 +/- 0.05 to 1.24 +/- 0.09 ml/min, respectively). Although absolute proximal sodium, bicarbonate, and chloride reabsorption increased, the increment in reabsorption was less than the increase in load; therefore solute delivery out of the proximal tubule increased by approximately 30-35%. Whole-kidney electrolyte excretion also rose markedly. When single-nephron and whole-kidney GFR were reduced back to a control level by aortic constriction (40.2 +/- 1.8 nl/min and 0.93 +/- 0.05 ml/min, respectively), proximal transport reverted to normal values despite persistent ANF administration. Ninety percent of the ANF-induced natriuresis and chloruresis were simultaneously abolished when GFR was normalized. In conclusion, ANF has no direct effect on reabsorption in the superficial proximal convoluted tubule independent of changes in filtration rate. Although direct effects on more distal or deeper nephron segments are not precluded, the present data suggest that ANF can increase renal solute excretion predominantly by acutely augmenting GFR.