To determine whether myocardial dysfunction contributes to vascular collapse in anaphylactic shock, we examined left ventricular (LV) contractility, coronary blood flow, and myocardial lactate metabolism during antigen challenge in eight dogs that were sensitized to ragweed pollen extract (anaphylaxis group). Findings in the anaphylaxis group were contrasted to those in another group of dogs in which mean blood pressure was decreased to the same extent by arteriolar vasodilation with nitroprusside. The animals were examined under nonhypoxic conditions while anesthetized and ventilated. LV mechanics were examined with subendocardial crystals placed primarily along the anterior-posterior minor axis of the LV. During antigen challenge, a depression in LV contractility was observed in the anaphylaxis group as assessed by fractional dimensional shortening, stroke volume, and the slope of the end-systolic pressure-dimension relationship. During anaphylaxis, moreover, coronary vasodilation rather than coronary vasoconstriction was observed, and evidence of myocardial ischemia as assessed by altered myocardial lactate metabolism was not found. Our results indicate that depressed LV contractility occurs in anaphylactic shock. The results further suggest that the mechanism may be due to a direct effect of mediators of anaphylaxis on the myocardium to produce systolic dysfunction.