Cimetidine, a drug in widespread use in the treatment of peptic ulcer disease, is eliminated primarily via urinary excretion. We examined cimetidine transport by rabbit proximal straight tubules perfused in vitro. [3H]Cimetidine in the bath was actively secreted into the tubule lumen. There was a curvilinear relationship between the rate of cimetidine secretion and the concentration of bath cimetidine. Cimetidine secretion was inhibited by hypothermia and ouabain. Quinine, tolazoline, probenecid, phloridzin, creatinine, p-aminohippurate, and cimetidine sulfoxide inhibited cimetidine secretion in a dose-related manner. At low cimetidine concentrations lumen-to-bath transport rates were only 11-18% of bath-to-lumen secretory rates. High performance liquid chromatographic analysis of collected tubular fluid showed a predominance of cimetidine and a small amount of cimetidine sulfoxide in ratios similar to those of the bath. These studies show that cimetidine is actively secreted into the lumen of rabbit proximal straight tubules in vitro. Secretion probably occurs via the organic base and to a lesser extent the acid transport systems.