The contribution of preglomerular resistance changes to the tubuloglomerular feedback (TGF) effector response was evaluated by micropuncture in hydropenic rats. Studies were performed in free-flowing nephrons to compare the fractional compensation (C) for perturbations in late proximal flow (VLP) with associated changes in glomerular capillary hydrostatic pressure (PGC). VLP was monitored by a noninvasive optical technique, and PGC was monitored by direct capillary micropuncture. Data were employed in conjunction with a model of glomerular filtration to analyze the TGF effector mechanism. C varied with the applied perturbation (VH), showing a single peak near the natural operating point. In contrast, the effect of TGF on PGC was asymmetric about the operating point, such that PGC was sensitive only to large positive perturbations. The model predicts that changes in preglomerular resistance account for only a small fraction of the integrated TGF response to small disturbances in the ambient state of the nephron but account for a greater share of the response to larger increments in flow. The TGF response in hydropenic rats is mediated by both pre- and postglomerular vascular elements.