Rabbit stomach smooth muscle contains mRNA for an internal Ca2+ pump identical in sequence to that reported for rabbit uterus [Lytton, Zarain-Herzberg, Periasamy & MacLennan (198) J. Biol. Chem. 264, 7059-7065]. This is an alternatively spliced form (Is) of the cardiac muscle sarcoplasmic reticulum Ca2+ pump (Ic). The splicing results in replacement of the last 4 amino acids (Ala-Ile-Leu-Glu) present in Ic by 49 amino acids and by a different 3′-non-coding region. Using cDNA probes against the conserved and the alternatively spliced regions, we determined that poly(A+) RNA isolated from rabbit stomach smooth muscle did not contain any transcripts for Ic. The poly(A+) RNA from cardiac muscle contained transcripts mostly for Ic, but also some for Is. The abundance of the Ca2(+)-pump transcripts as measured by the binding of a cDNA probe against the conserved region to poly(A+) RNA was 6-8 times higher in cardiac than in smooth muscle. The amount of the corresponding pump protein, measured using two antibodies, was 60-80 times higher in cardiac membranes than in smooth muscle membranes. Thus the protein-to-transcript level was approx. 10-fold higher in the cardiac muscle. We conclude that the regulation of the abundance of this protein occurs at steps leading to the formation of the mature mRNA for the two splices, which may differ in their translation efficiency.