The effect of dietary fish oil on cardiac function and responsiveness to alpha- and beta-adrenergic receptor agonists was examined in isolated perfused rat hearts. Rats were fed either a standard laboratory diet (SD) or diets containing 5% corn oil (CO) or 5% menhaden oil (MO) for 4 wk. When perfused as working preparations at varying preloads and afterloads, the peak aortic pressures, aortic outputs, and coronary flows were comparable in hearts of rats fed the three experimental diets. Inotropic responsiveness to phenylephrine was examined by infusing graded doses of the drug into the heart while monitoring changes in the rate of left ventricular pressure development (+dP/dt). Prior to phenylephrine administration +dP/dt was not different among the three groups of hearts. However, at each dose of phenylephrine employed, delta +dP/dt was approximately 50% less in hearts of rats fed MO when compared with either SD or CO. Thus cardiac inotropic responsiveness to this alpha-agonist was reduced by dietary fish oil. In contrast, cardiac inotropic responsiveness to isoproterenol was not altered with MO feeding. The data demonstrate that dietary fish oil results in alterations in alpha- but not beta-adrenoceptor mediated changes in cardiac inotropy.