We have shown previously that Leu447 and Gln448 in the transmembrane helix (TMH) 10 of rat organic cation transporter rOCT1 are critical for inhibition of cation uptake by corticosterone. Here, we tested whether the affinity of corticosterone is different when applied from the extracellular or intracellular side. The affinity of corticosterone was determined by measuring the inhibition of currents induced by tetraethylammonium(+) (TEA(+)) in Xenopus laevis oocytes expressing rOCT1. Either corticosterone and TEA(+) were added to the bath simultaneously or the oocytes were preincubated with corticosterone, washed, and TEA(+)-induced currents were determined subsequently. In mutant L447Y, K(i) values for extracellular and intracellular corticosterone were decreased, whereas in mutant Q448E, only the K(i) for intracellular corticosterone was changed. Modeling of the interaction of corticosterone with rOCT1 in the inward- or outward-facing conformation predicted direct binding to Leu447, Phe160 (TMH2), Trp218 (TMH4), Arg440 (TMH10), and Asp475 (TM11) from both sides. In mutant F160A, affinities for extracellular and intracellular corticosterone were increased, whereas maximal inhibition was reduced in W218F and R440K. In stably transfected epithelial cells, the affinities for inhibition of 1-methyl-4-phenyl-pyridinium(+) (MPP(+)) uptake by extracellular and intracellular corticosterone were decreased when Asp475 was replaced by glutamate. In mutants F160A, W218Y, R440K, and L447F, the affinities for MPP(+) uptake were changed, and in mutant D475E, the affinity for TEA(+) uptake was changed. The data suggest that Phe160, Trp218, Arg440, Leu447, and Asp475 are located within an innermost cavity of the binding cleft that is alternatingly exposed to the extracellular or intracellular side during substrate transport.