Micropuncture experiments were conducted to assess autoregulatory behavior of superficial nephrons and to evaluate the possible contribution of alterations in tubular and peritubular capillary pressures to the mechanism responsible for the arterial pressure effects on urine flow and sodium excretion. In response to decreases in renal arterial pressure (RAP), autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR) was highly efficient to blood pressures as low as 80 mm Hg. Proximal tubule pressure (PTP), peritubular capillary pressure (PCP), distal tubule pressure, and single nephron GFR (SNGFR) remained within 5% of control values with reduction in RAP to 90 mm Hg. Further decreases in RAP to the lower autoregulatory range caused significant decreases in PTP and PCP; the slight decreases in SNGFR were not significant. Within the autoregulatory range, GFR and filtered sodium load were not altered, and urine flow and sodium excretion responses were due to changes in net fractional sodium and water reabsorption. No quantitative relationships could be established between the magnitude of the changes in urine flow or sodium excretion and PTP, PCP, or proximal tubule fractional reabsorption. These experiments demonstrate that superficial nephrons autoregulate in association with the total nephron population except for a proportionately greater reduction when RAP is reduced to the lower autoregulatory range. Furthermore, the urinary responses to reduced arterial pressure occur even in the absence of quantitatively associated alterations in proximal tubule function or PCP.