We investigated whether a nitric oxide synthase (NOS) inhibitor improves cardiovascular depression associated with anaphylaxis. After induction of anaphylactic circulatory depression, one group received an NOS inhibitor (Group I, n= 6) and the other received saline solution (Group II, n= 5). Mean arterial pressure and right atrial pressure were significantly higher in Group I than in Group II. Hematocrit was significantly lower in Group I than in Group II. Cardiac output, stroke volume, mean pulmonary arterial pressure, the maximum rate of increase in left ventricular pressure, and the time constant of the fall in isovolumic left ventricular pressure did not differ between the groups. In conclusion, L-NAME attenuates hypotension, but does not improve cardiac depression in anaphylaxis in dogs. Our finding that NOS inhibitor did not improve cardiac function implies that the production of NO in anaphylaxis may have a protective effect with regard to cardiac performance.