Using conventional micropuncture techniques in 53 adult Sprague-Dawley rats anesthetized with sodium pentobarbital, single nephron GFR at proximal sites (snGFRprox) averaged 30+/-1 nl/min (76 tubules) and at distal sites (snGFRdis), 33+/-2 nl/min (90 tubules). Thus, in spite of interruption of macula densa fluid flow during its measurement, snGFRprox was not significantly greater than snGFRdis (p less than 0.05). Whole kidney GFR averaged 0.9+/-0.07 ml/min before and 1.0+/-0.05 ml/min during an intravenous infusion of SQ20,881, a substance which inhibits production of angiotensin II. Similarly, snGFRdis was unaffected by SQ20,881; the mean difference between paired snGFRdis (collection during NaCl infusion minus recollection at same sites during SQ20,881 infusion) was 6+/-4 nl/min (p less than 0.5). These results suggest that under the conditions of the experiments, tubulo-glomerular feedback is not operative and that angiotensin II plays no role in controlling filtration.