SummaryThe effects of postnatal development on the systolic and diastolic responses to pharmacologic blockade of the slow inward calcium current were investigated in blood‐perfused hearts isolated from immature (14‐21‐day‐old) and adult rabbits. Isovolumic left ventricular developed pressure, resting pressure, and maximal rate of pressure development (+dP/dt) at cumulative doses of either verapamil, nifedipine, or diltiazem were determined by means of an intracavitary balloon. Myocardial contractile function in the immature heart was more sensitive to pharmacologic blockade of the slow inward calcium current than is the adult heart. Doses of verapamil, or nifedipine, that comparably reduced pretreatment developed pressure and +dP/dtwere approximately 10‐fold less in immature as compared to the adult heart. The dose of diltiazem which reduced developed pressure anddP/dtby 50% was 3‐fold less in immature as compared to the adult heart. Verapamil and nifedipine decreased resting pressure in the adult but not in the immature heart. Conversely, diltiazem decreased resting pressure in the immature while not affecting resting pressure in adult hearts. Thus, postnatal cardiac development affects both the systolic and diastolic responses to calcium channel blockade. In addition, diltiazem appears to be qualitatively and quantitatively different from verapamil and nifedipine with respect to the age‐related cardiac effects of calcium channel blockade.AbbreviationsSR, sarcoplasmic reticulumSL, sarcolemmaLV, left ventricular