The effect of halothane on Ca2+ uptake or release by the sarcoplasmic reticulum (SR) was compared in the newborn and adult rabbit myocardium. The sarcolemma of right ventricular myocardium was disrupted (skinned) by homogenization. Fiber bundles were dissected from the homogenate, mounted on tension transducers, and immersed sequentially in five solutions that loaded Ca2+ into the SR, then in solutions containing either 2 or 25 mM caffeine to release SR-stored Ca2+, resulting in transient tension development. Experimental solutions were saturated with halothane in N2 gas during Ca2+ uptake by SR, Ca2+ release by SR, or during both SR Ca2+ uptake and release. Halothane (0.5-1.7%) resulted in dose-dependent depression of SR Ca2+ uptake in both newborn and adult skinned fibers. Less tension transient depression resulted in newborn (35%) than adult skinned fibers (49.5%, P less than 0.05) with 0.5% halothane exposure during SR Ca2+ uptake. Similar depression resulted in newborn (53.7% and 73.4%) and adult fibers (65.2% and 77.9%) with 1.0% and 1.7% halothane. Halothane had little effect on SR Ca2+ release by 25 mM caffeine but enhanced submaximal SR Ca2+ release by 2 mM caffeine more in newborn than adult myocardium. Increased Ca2+ efflux from newborn SR may contribute to the greater sensitivity of intact newborn cardiac muscle to exposure to halothane.