The objective of these experiments was to test the hypothesis that renal interstitial hydrostatic pressure (RIHP) plays an important role in the natriuretic effect of atrial natriuretic factor (ANF) in anesthetized Wistar rats. Three groups of male Wistar rats were used in this study. Two groups were infused with different doses of ANF, and the third group was a time control. In all groups, one kidney was acutely decapsulated, and the contralateral kidney was used as control. Renal decapsulation was used to control RIHP. In one group, 3 micrograms.kg-1.h-1 of synthetic ANF were infused intravenously (iv) and produced a plasma level of ANF (PANF) of 810 +/- 186.5 pg/ml. This pharmacological dose of ANF produced a significant increase in RIHP of the control kidney from 9.5 +/- 0.8 to 11.1 +/- 1.3 mmHg (P less than 0.05) but not in the decapsulated kidney [from 7.1 +/- 0.6 to 8.1 +/- 0.9 mmHg, not significant (NS)]. However, the changes in fractional excretion of sodium (FENa) and urine flow rate (V) as a result of ANF infusion were similar in both kidneys. In the decapsulated kidney, FENa and V increased by 1.53 +/- 0.41% and 26.21 +/- 5.98 microliters/min, respectively, from control to ANF infusion periods. In the control kidney, FENa and V increased by 1.60 +/- 0.28% and 31.61 +/- 5.87 microliters/min, respectively, from control to ANF infusion periods. In the second group, 1 microgram.kg-1.h-1 iv of synthetic ANF was infused and produced 165.2 +/- 29.3 pg/ml of PANF.(ABSTRACT TRUNCATED AT 250 WORDS)