The effects of substitution of calcium (Ca) by an equimolar concentration of strontium (Sr) on isometric contractions of isolated ventricular muscle from postnatally developing rat heart were studied. The duration of contraction and the time-to-peak tension were increased in all age groups although much less in the adult rats than in the neonates. The contractile force was increased in the muscles of rats between 1 and 14 days of age but was depressed in the older animals. The prominent rest-twitch potentiation of neonatal rat heart in Ca− Tyrode was totally eliminated by Sr, whereas a clear rest-twitch potentiation was induced by this cation in the adult rat heart, in which tissue the potentiation is normally absent in Ca− Tyrode. The maximal twitch potentiation by rest in Ca− Tyrode and the positive inotropic effect of Sr substitution grew from birth up to day 9 and from then gradually declined towards the level of adult rat heart by the end of the 3rd postnatal week. The phase of increasing rest-twitch potentiation coincides fairly well with the known development of sarcoplasmic reticulum and the phase of decline with the appearance of the T system of the sarcolemma. It is suggested that the qualitative changes in the contractile properties of developing rat heart during the 3rd postnatal week are due to the more efficient utilization of intracellular calcium stores, owing to the development of the T system.