Experiments were performed to evaluate the influence of the distal tubular feedback mechanism on glomerular function in the dog. Single nephron glomerular filtration rate (SNGFR) and stop-flow pressure (SFP) were measured from early segments of proximal tubules during alterations in distal perfusion rate (via a late proximal puncture site) and perfusate composition. Perfusion rate (PR) was varied from 16 to 68 nl/min with a microperfusion pump. The intermediate segment of the nephron was blocked with a solid wax cast, thus preventing retrograde influences of the microperfusion procedure. During perfusion with an ultrafiltrate of plasma and an artificial tubular fluid solution (ATF), SNGFR decreased from 63 ± 2.6 (SE) nl/min at a PR of 16 nl/min to 20 ± 2.4 nl/min at a PR of 63 ± 2.6 nl/min. At a PR of 16 nl/min, SFP was 48 ± 1.3 mmHg with ultrafiltrate and ATF. Increases in PR to 68 nl/min led to a reduction in SFP to 26 ± 1.5 mmHg. SFP was also measured during changes in PR with various electrolyte solutions of decreasing complexity. Elimination or substitution of Na+, K+, Cl-, HCO3-, and Ca2+did not significantly alter the magnitude of the feedback response to increases in PR to 68 nl/min. These results confirm the existence of a feedback system in the dog capable of adjusting glomerular function in response to changes in distal perfusion rate. The results based on the various perfusion solutions fail to indicate a unique requirement for any specific component.