This study determines the inotropic effects of prostaglandin D2(PGD2) and prostaglandin E1(PGE1) in the isolated, arterial perfused newborn (NB) and adult (A) rabbit heart.Significant positive inotropism of PGD2was observed at all concentrations studied (1 x 10-17to 1 x 10-7M) in the two age groups; the effect in the NB was significantly greater (p<0.05) than that in the A at PGD2concentrations higher than 1 X 10-17M. Significant positive inotropism of PGE1was observed at PGE1concentrations higher than 1 x 10-8M in the NB, and only at 1 x 10-6M in the A.In the NB, the relaxation parameters [1/2ART and the ratio of +dT/dt (max) to -dT/dt (max)] decreased to 80% of control after PGE1infusion, but not after PGD2infusion. In contrast, relaxation parameters in the A were not different from control.Propranolol (1 x 10-6M) did not alter the positive inotropic action of PGD2and PGE1in the NB. These data indicate that: 1) the positive inotropic effects of PGD2and PGE1in NB are greater than that in the A, 2) PGE1and not PGD2, enhances myocardial relaxation only in the NB,3) the contractile effects of PGD2and PGE1are not mediated by β-receptors.