Patch-clamp experiments in both clonal GH3 cells and guinea-pig bladder myocytes reveal that 1-benzyl-7-chloro-4-(n-pentylimino)-1,4-dihydroquinoline hydrochloride (WIN 17317-3), a potent blocker of Kv1.3 channels and potential immunomodulator, reduces, in a reversible manner and at low micromolar concentrations, K+ currents through Ca2+-activated high conductance K+ channels (BK channels). Blockade of BK channels is thought to account for the stimulatory effect of WIN 17317-3 on the contractility of guinea-pig bladder. This effect is not modified by tetrodotoxin (1 microM), but is abolished by nifedipine (0.1 microM). In conclusion. WIN 17317-3 lacks selectivity for the Kv1.3 channels, its postulated target for immunosuppression.