Sarcoplasmic reticulum Ca²⁺‐ATPase cDNA clones have been isolated from an adult rat heart cDNA library and the nucleotide sequence of the Ca²⁺‐ATPase mRNA determined. The sequence has an open reading frame of 997 codons. It is identical to a cDNA isolated from a rat stomach cDNA library and 90% isologous to the rabbit and human slow/ cardiac cDNAs. Nuclease S₁ mapping analysis indicates that this sequence corresponds to the main Ca²⁺‐ATPase mRNA present in heart and in slow skeletal muscle and that it is expressed in various proportions in smooth and non‐muscle tissues, together with another isoform which differs from the cardiac form in the sequence of its 3′‐end.