Experiments were conducted to study the transport of the histamine H2-receptor antagonist, cimetidine, in luminal membrane vesicles prepared from rabbit renal cortex. Cimetidine accumulated in the vesicles with time. Cimetidine uptake was sensitive to changes in vesicle size, suggesting that the compound is transported into an osmotically reactive intravesicular space. Its rate of uptake could be described by both a saturable and a nonsaturable process. The Km was 4.6 +/- 4.0 microM and the Vmax was 6.8 +/- 2.3 pmol X s-1 X mg protein-1 (mean +/- SD, n = 4). N1-methylnicotinamide (NMN), cimetidine, cimetidine sulfoxide, and ranitidine inhibited the uptake of cimetidine. Cimetidine uptake in the presence of an outwardly directed proton gradient was enhanced in vesicles preloaded with a higher concentration of unlabeled cimetidine (2.4 X 10(-4) M). An outwardly directed proton gradient enhanced the uptake of cimetidine to values exceeding its equilibrium accumulation. Uptake stimulated in this way could be inhibited by the cation, NMN, the bases, ranitidine, and cimetidine sulfoxide, and interestingly, by the anion, probenecid. The effect of probenecid did not appear to be due to nonspecific effects on membrane binding, membrane potential, or vesicle size. These data are consistent with data obtained in isolated perfused proximal tubules, demonstrating that probenecid inhibits cimetidine transport. The data in this study suggest that the effect of probenecid on cimetidine transport specifically involves the transporter in the luminal membrane.