Ca2+-uptake activities of the sarcoplasmic reticulum (SR) were determined with a Ca2+ -sensitive electrode in homogenates from fast- and slow-twitch muscles from both normal and dystrophic mice (C57BL/6J strain) of different ages. Immunochemical quantification of tissue Ca2+-ATPase content allowed determination of the specific Ca2+ -transport activity of the enzyme. In 3-week-old mice of the dystrophic strain specific Ca2+ transport was already significantly lower than in the normal strain. It progressively decreased with maturation and reached only 40–50% and 30–50% of the normal values in fast- and slow-twitch muscles of adult dystrophic animals, respectively. Tissue contents of calsequestrin were reduced in both types of muscle leading to an increased Ca2+-ATPase to calsequestrin protein ratio. Equal amounts of the Ca2+ -ATPase protein (detected by Coomassie blue staining of polyacrylamide gels) were present in SR vesicles isolated by Ca2+ -oxalate loading from adult normal and dystrophic fast-twitch muscles. However, the specific ATP-hydrolysing activity of the enzyme was approximately 50% lower in dystrophic than in normal SR. The reduced ATP-hydrolysing activity was correlated with decreased Ca2+ -transport activity, phosphoprotein formation and fluorescein isothiocyanate labeling as determined in total microsomal and heavy SR fractions. Although the Ca2+ and ATP affinities of the enzyme were unaltered, its ATPase activity was reduced at all levels of ATP in the dystrophic SR. Taken together, these findings point to a markedly impaired function of the SR and an increase in the population of inactive SR Ca2+ -ATPase molecules in murine muscular dystrophy.