Transepithelial transport of tetraethylammonium (TEA) was studied in monolayers of opossum kidney cells cultured on permeable membrane filters. [¹⁴C]-TEA was transported across the OK cell monolayer from basolateral to apical side by a saturable process which can be stimulated by acidification of the apical medium. The apparent Michaelis-Menten constant (K_m) and the maximum velocity(V_max) for the transport were 41 μM and 147 pmole/ mg protein/ min, respectively. The transport was significantly inhibited by unlabelled TEA, amiloride, cimetidine, choline, and mepiperphenidol added to the basolateral side at 1 mM and was slightly inhibited by 5 mM N₁-methylnicotinamide (NMN). Unlabelled TEA added to the apical side stimulated the basolateral-to-apical {¹⁴C}-TEA transport, suggesting that the TEA self-exchange mechanism was involved at the apical membrane. Sulfhydryl reagents such as {\rho}-chloromercuribenzoic acid (PCMB) and ρ-chloro-mercuribenzene sulfonate (PCMBS) and carboxyl reagents such as N,N -dicyclohexylcarbodiimidem (DCCD) and N-ethoxy-carbonyl-2-ethoxy-1,2-dihydro-quinoline(EEDQ) inhibited the TEA transport at both the basolateral and apical membranes of the OK cell monolayer. These results suggest that OK cell monolayers possess a vectorial transport system for organic cations which is similar to that for organic cation secretion in the renal proximal tubule.