Substrate specificity and pH dependence of the transport system for diphenhydramine were investigated in Caco-2 cell monolayers. Diphenhydramine uptake was not affected by any typical substrate for the renal organic cation transport system except procainamide. Along with procainamide, tertiary amine compounds with N-dimethyl or N-diethyl moieties in their structures inhibited the diphenhydramine uptake. Moreover, accumulation of diphenhydramine was stimulated by preloading the Caco-2 cells with these tertiary amines (trans-stimulation effect), indicating the existence of the specific transport system for tertiary amines with N-dimethyl or N-diethyl moieties. Efflux of diphenhydramine from monolayers was enhanced by medium acidification. In addition, intracellular acidification resulted in marked stimulation of diphenhydramine accumulation. ATP depletion of the cells caused an enhancement of diphenhydramine accumulation, suggesting the involvement of an active secretory pathway. However, P-glycoprotein did not mediate the diphenhydramine transport. These findings indicate that a novel pH-dependent tertiary amine transport system that recognizes N-dimethyl or N-diethyl moieties is involved in diphenhydramine transport in Caco-2 cells.