To determine whether the changes in single nephron vascular resistance that follow increased rates of flow through the loop of Henle of the same nephron occur at a pre- or a postglomerular site, the effect of increased loop flow on single nephron filtration fraction (SNFF) was measured. Anatomical criteria were developed to identify surface stellate vessels and proximal tubule segments belonging to the same nephron in rats. The criteria were verified by demonstrating that the usual increase in hematocrit (Hct) between systemic artery and stellate vessel could be prevented by placement of an immobile block in the associated proximal tubule. Stellate vessel pressure (PSV), stop-flow pressure (PSF), tubule fluid flow rate, tubule fluid inulin concentration, and stellate vessel Hct were measured in vascular or tubular segments, while the loop of Henle of the same single nephron was perfused alternately at normal and high rates. Values were calculated for single nephron glomerular filtration rate (SNGFR) and SNFF. In association with increased rates of loop flow, SNFF decreased 24%, SNGFR decreased 35%, and PSF decreased 20%. PSV did not change. The magnitude of this change in SNFF, when compared to the changes in SNGFR and PSF, indicates that the feedback response to increased loop of Henle flow is probably mediated primarily by afferent arteriolar vasoconstriction.