The present study was undertaken to clarify the effect of the first generation histamine H1 receptor antagonists on the adenosine triphosphate-sensitive potassium (KATP) channel in the isolated mouse ventricular cardiomyocytes using excised inside-out and cell-attached patch clamp techniques. In the excised inside-out patch configuration, H1-antihistaminic agents (chlorpheniramine, pyrilamine and diphenhydramine), doses ranging from 1 to 100 μmol/L, inhibited KATP channel activity in a dose-dependent manner. The potency order reducing the channel activity was pyrilamine > diphenhydramine > chlorpheniramine. All the antihistamines (100 μmol/L) also inhibited the pinacidil-induced KATP channel activity in the cell-attached patch configuration. The potency order of the 3 antihistamines inhibiting KATP channel activity is pyrilamine > chlorpheniramine > diphenhydramine in the cell-attached configurations. Histamine did not affect the pinacidil-induced KATP channel activities by itself, in addition, did not influence the effects elicited by the 3 antihistamines on the pinacidil-induced KATP channel acitivities in the cell-attached patches. From these results, it concluded that the first generation histamine H1 receptor antagonists are involved in the regulation of the ATP-sensitive potassium channel activities in the mouse cardiac ventricular myocytes, and that the inhibitory actions of the antihistaminic agents on the channels are not dependent on the H1-receptors.