Each of three harmala alkaloids, harmine, harmaline, and harmalol, decreased heart rate and increased pulse pressure, peak aortic flow, and myocardial contractile force in intact normotensive anesthetized dogs. Harmine reduced systemic arterial blood pressure and total peripheral vascular resistance; harmaline-evoked decreases were frequently followed by a secondary increase; and the effects of harmalol on these two parameters were inconsistent. A direct negative chronotropic effect of harmala alkaloids was suggested by observations of bradycardia in the isolated perfused rat heart and in the intact dog; neither vagotomy nor atropinization affected harmala alkaloid-induced bradycardia in the dog. Reduction in femoral vascular resistance by the alkaloids was not apparently due to activation of cholinergic, beta-adrenergic, or histamine (H1) receptors.