Noradrenaline-β1 adrenoceptor regulates contraction of mammalian heart. However α-adrenoceptor-mediated regulation of heart contractility has been reported in several species. Aim of the present study is to determine function and expression of α-adrenoceptors in the mouse atrium using functional and molecular biological approaches. In the spontaneous beating right atrium, noradrenaline and phenylephrine caused positive inotropic and positive chronotropic actions. On the other hand, clonidine and xylazine caused positive inotropic actions and negative chronotropic actions at high concentrations. Phenylephrine-induced positive inotropic and chronotropic actions were partially decreased by propranolol, and both actions remained in the presence of propranolol were inhibited by prazosin. A low concentration of silodosin (< 100 nM) did not but a high concentration (1 μM) decreased the phenylephrine-induced chronotropic actions. Neither propranolol nor phentolamine decreased the negative chronotropic actions of clonidine and xylazine. Expression of α1B mRNA was the highest among α1-adrenoceptors (α1B＞α1A=α1D) in the atrium. In conclusion, α1B-adrenoceptors are dominant α1-adrenoceptor subtypes and regulate the contraction of mouse atrium.