The effect of dopamine on coronary vascular resistance and myocardial oxygen consumption was investigated. Spontaneously-beating, paced, K+-arrested and fibrillating hearts were used. Dopamine caused either an increase or a decrease in coronary vascular resistance depending upon the dose of dopamine used and whether or not the positive inotropic effect of the drug was allowed to develop. The coronary constrictor effect of dopamine was blocked by phenoxybenzamine but persisted during β-blockade.