Rates of isolated embryonic chick hearts (ECH) and embryonic rat hearts (ERH) of various ages were maintained constant by field stimulation and the characteristics of their inotropic responses to isoproterenol (Iso) were investigated. Exposure to Iso produced concentration, age, species and calcium dependent increases in contractility that were prevented by propranolol (3x10(-6) M). The slopes of the linear portions of the concentration-response curves increased with age (14-20 days) in the ERH, the maximal slope and inotropic effect being observed in the 18-day-old heart, whereas they decreased with age (3-7 days) in the ECH, the minimal slope but maximal inotropic effect being observed in the 5-day-old heart. At 10(-7) M, Iso produced approximately maximal responses at all ages in the ECH but elicited only minimal responses at all ages in the ERH, approximately ten times this concentration being required to produce maximal responses in the ERH. Reducing the calcium concentration of the bathing medium significantly increased the sensitivities of the hearts of both species to the inotropic effects of Iso. Iso-induced positive inotropic responses were also demonstrated in isolated, driven ventricles from 4-day-old ECH. These results indicate that functional, beta-adrenergic, inotropic receptors are present in the embryonic hearts of both species and undergo changes in responsiveness during embryonic development which are species-dependent.