Tetraethylammonium (TEA)-sensitive potassium currents in the cochlear inner hair cells (IHCs) possess the kinetics of fast inactivation. IHCs of guinea-pigs were separately isolated from the apical and basal turns and the tonotopic gradient of inactivation kinetics was investigated. TEA-sensitive potassium currents showed voltage-dependent time constant of the inactivation phase both in apical and basal IHCs, however, the degree of inactivation (compared to the ratio between the steady-state current and initial peak current) was voltage-independent. Inactivation time constant was faster in basal IHCs than in apical IHCs and the degree of inactivation was greater in basal IHCs than in apical IHCs, suggesting that inactivation was more predominant in basal IHCs than in apical IHCs.