The effect of atrial natriuretic factor [ANF-(1-28); 0.25 μg·kg-1·min-1] on tubuloglomerular feedback (TGF) and the efficiency of renal blood flow (RBF) autoregulation was determined in anesthetized euvolemic rats. In microperfusion studies, ANF dramatically inhibited (>80%) feedback-mediated decreases in single-nephron glomerular filtration rate (SNGFR) and stop-flow pressure (P(sf)) when Henle's loop was perfused at 0-48 nl/min with artificial fluid. The sigmoidal relationship between P(sf) and loop perfusion during control was shifted to a linear relation during ANF; reactivity was almost nonexistent and no inflection point could be discerned. ANF almost completely blocked maximum P(sf) and SNGFR responses to loop perfusion at rates >24 nl/min. In contrast 30 nl/min loop perfusion with native proximal tubular fluid obtained during ANF infusion restored maximum TGF activity to 70% of control levels. During ANF administration, the paired P(sf) responses to native and artificial perfusate were significantly different (-5.3 vs. -0.8 mmHg, P < 0.001), compared with similar responses during control conditions (-7.6 vs. -8.3 mmHg, P > 0.1). In free-flow studies, ANF increased proximally and distally measured SNGFR equally. The constancy of the proximal-distal SNGFR difference (10.3 vs. 9.3 nl/min) in the presence of increased distal fluid delivery suggests partial inhibition of TGF during ANF administration. ANF elevated P(sf) but did not affect basal RBF or the RBF autoregulatory index over an arterial pressure range of 130-70 mmHg. These results indicate that 1) RBF autoregulation is efficiently maintained during ANF infusion when preglomerular vessels are vasodilated and TGF is inhibited by ~30%; 2) an endogenous factor(s) in native proximal tubular fluid may attenuate ANF-induced inhibition of TGF; and 3) microperfusion studies using artificial fluid significantly overestimate the net in vivo effect of ANF on TGF.