The hemodynamic effects of octopamine (OCT) were studied before and during dopamine (DA) infusion in six normal dogs. Before OCT infusion, DA significantly increased cardiac index (CI) from 3.24 ± 0.20 to 4.90 ± 0.30 L/min - m2 and significantly decreased systemic vascular resistance index (SVRI) from 3700.2 ± 212.5 to 2618.3 ± 156.6 dyne · sec/cm5 · m2, without changing heart rate. During OCT infusion, DA failed to increase CI or decrease SVRI; however, it significantly increased heart rate from 153.3 ± 12.9 to 183.0 ± 17.0 beat/min. This suppression of a DA-related increase in cardiac output may explain why DA is ineffective in those septic patients with elevated plasma OCT levels.