This study compared the effects of high-dose infusions of various adrenergic agonists on cardiovascular function in piglets. We hypothesized that agonists would have different effects on systolic, diastolic, and vascular functions. Nine anesthetized 3-week-old piglets underwent cardiac catheterization. Manometric and conductance catheters measured pressures and volumes. Data were acquired at rest and during infusions of epinephrine, norepinephrine, dopamine, dobutamine, isoproterenol, and phenylephrine. End-systolic elastance, preload-recruitable stroke work, cardiac output, the maximum and minimum derivatives of left ventricular pressure, the relaxation constant tau, peak filling rate, and end-diastolic stiffness were obtained. Contractile efficiency and the cardiac output/pressure-volume area ratio were calculated. Regression was used for analysis of variance; p < 0.05 was considered significant. All agonists increased indexes of contractility. beta-Adrenergic agonists enhanced relaxation. Isoproterenol and dopamine increased efficiency. No drug changed diastolic stiffness. Therefore both alpha-adrenergic and beta-adrenergic agonists have inotropic effects in the 3-week-old piglet. Some beneficial effects of beta-agonists on cardiac output may be due to enhancement of relaxation and to afterload reduction. Various agents exert different effects on the cardiovascular system, and these differences may be clinically important.