Temporal patterns of biosynthesis of the high affinity calcium binding protein from the sarcoplasmic reticulum were determined and compared with rates of ATPase ane cells. Cells at various stages of differentiation were incubated for 2 h with [35S]methionine. Specific proteins were isolated from detergent extracts of cells by incubation with antibodies specific against the various proteins and immunoprecipitates were separated by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. Radioactivity incorporated into specific bands was analyzed by counting gel slices and incorporation data were used to obtain relative rates of individual protein sys found to be indistinguishable from that of calsequestrin when cells were grown in standard medium, in medium containing 60 microM Ca2+ which prevented fusion of cells, or in enriched medium which delayed cell fusion. The high affinity calcium binding protein had a relatively high turnover rate with a half-life of about 10 h. These studies suggest that synthesis of calsequestrin and the high affinity calcium binding protein are coordinated even though calsequestrin is a glycoprotein, whereas the high affinity calcium binding protein is not glycosylated.