Using conventional micropuncture techniques in 53 adult Sprague-Dawley rats anesthetized with sodium pentobarbital, single nephron GFR at proximal sites (snGFR<sub>prox</sub>) averaged 30 ± 1 nl/min (76 tubules) and at distal sites (snGFR<sub>di S</sub>), 33 ± 2 nl/min (90 tubules). Thus, in spite of interruption of macula densa fluid flow during its measurement, snGFR<sub>pro X</sub> was not significantly greater than snGFR<sub>di S</sub> (p > 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, snGFR<sub>di S</sub> 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 > 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.