The pulse-injection multiple-indicator-dilution technique in vivo has been used to investigate bidirectional sugar interaction with the antiluminal surface of the nephron in dog kidney. Simultaneous renal vein and urine outflow curves were obtained for radiolabeled sugars known to interact with the antiluminal surface. The following sugars were tested relative to T-1824 albumin (plasma reference) and creatinine (extracellular reference) under conditions of high-dose phlorizin preloading (75-225 mg/kg): D-glucose, D-xylose, D-fucose, D-mannose, D-galactose, L-arabinose, myoinositol, and D-fructose. The results indicate that as the plasma concentration of phlorizin increases there is, first, a partial inhibition of sugar interaction at the antiluminal membrane so that only unidirectional uptake of sugar from blood to tubular cell is observed, followed by complete inhibition of sugar interaction at the peritubular face of the antiluminal membrane, resulting in superposition of sugar and creatinine curves in the renal vein effluent. Two possible interpretations exist. i) Phlorizin exerts its inhibitory action successively at the cytoplasmic and then at the peritubular face of the antiluminal membrane. Moreover, since all of the sugar substrates are inhibited by phlorizin, the data suggest that the sugar-membrane interaction takes place at a common site at the level of the proximal tubule. ii) Alternatively, the action of phlorizin could result from a metabolic inhibitory effect affecting multiple sugar transport systems at the antiluminal membrane.