We have recently found that mouse megakaryocytes responded to extracellular alkalinization to pH>8.0, generating a K⁺ current under voltage-clamped conditions with the whole cell recording mode of the patch-clamp technique. The purpose of this study was to physiologically and pharmacologically characterize the alkaline-dependent K⁺ conductance of the megakaryocyte membrane. The alkalinization-induced K⁺ current (I_ALK) did not seem to be Ca²⁺-dependent since I_ALK was allowed to be generated under intracellularly Ca²⁺-buffered conditions with 10 mM EGTA, which completely prevented the generation of caffeine-induced Ca²⁺-activated currents of mouse megakaryocytes; and no [Ca²⁺]_i elevation was evoked by the alkalinization protocol in contrast to a significant increase in [Ca²⁺]_i in response to caffeine when [Ca²⁺]_i was measured with a fura 2 ratiometry. I_ALK was strongly suppressed with tetraethylammonium (TEA), 4-aminopyridine (4-AP) and streptomycin (SM), but was completely resistant to quinidine (QND). The values of IC₅₀ for the suppression of I_ALK with TEA, 4-AP and SM were 5.6, 0.47 and 1.5 mM, respectively. Voltage-gated K⁺ currents (I_K) of the same megakaryocyte preparation were weakly suppressed with TEA and 4-AP, while they were significantly suppressed with either SM or QND. These results suggest that mouse megakaryocytes possess K⁺ conductance that was activated by extracellular alkalinization and that probably differs from conventional K⁺ conductance in its pharmacological properties.