The mechanisms involved in the excretion of the histamine H2-receptor antagonist cimetidine are as yet incompletely understood. The purpose of this study was to examine the interaction of cimetidine with incubated slices of dog kidney cortex. The results of time and concentration-dependent experiments by using 3H-labeled cimetidine demonstrated that the drug was accumulated without metabolism against a concentration gradient by a saturable process. Inhibition of uptake by cyanide and by incubation under a nitrogen atmosphere indicated energy dependence. Uptake of cimetidine by active cationic transport was likely inasmuch as both cyanine 863 and quinine blocked accumulation. However, a probenecid-sensitive component, accounting for about 20% of steady-state accumulation, also was identified. The lack of inhibitory action of p-aminohippuric acid and the cationic nature of the cimetidine molecule suggest the probenecid-sensitivity was not related to the renal organic anion mechanism. Further, probenecid inhibition was not due to a generalized cellular toxicity because maximally inhibitory concentrations of probenecid did not interfere with uptake of the cation tetraethylammonium.