The effects of histamine on inotropic and chronotropic activity were investigated in isolated canine atrium preparations which were suspended in a bath and perfused with arterial blood from a carotid artery of heparinized support dog. Histamine administered into the cannulated sinus node artery in a dose range of 0.3-100 .mu.g produced dose-related positive chronotropic and inotropic effects. Positive responses to histamine were not suppressed by treatment with propranolol in doses which blocked responses to norepinephrine, but were significantly suppressed by a histamine H1 receptor blocking agent, tripelennamine. These histamine-induced effects were not modified by a histamine H2 receptor blocking agent, burimamide. Positive chronotropic and inotropic effect of histamine may be mediated by histamine H1 receptors in the dog heart.