The characteristics of the inotropic responses to isoproterenol (Iso) were investigated in isolated hearts from embryonic normotensive (NT) and spontaneously hypertensive (SH) Wistar-Kyoto rats at 14, 16, 18 and 20 days of gestation and from rats 3 and 10 weeks after birth and in ventricular strips prepared from hearts of newborn and 1-week-old rats. With rates of all preparations maintained constant by electrical stimulation, concentration and age dependent increases in contractility in response to Iso, capable of being blocked by propranolol, were observed. In embryonic hearts, irrespective of Iso concentration or embryonic age, similar effects were observed in hearts from NT and SH animals. However, in all postnatal preparations, the effects of Iso on the SH strain were quantitatively different from those on hearts of NT rats of the same age. As early as 12 hr after birth and through 10 weeks of age, most concentrations of Iso induced inotropic effects that were significantly greater in hearts of the SH than the NT strain and the slopes of the linear portions of the concentration-response curves were significantly steeper. These findings indicate that functional beta adrenergic inotropic receptors are present in the embryonic hearts of both strains and undergo changes during development that are strain dependent.