The present study examined the level of the mRNA coding for the sarcoplasmic reticulum (SR) Ca(2+)-ATPase in isolated ventricular myocytes from streptozotocin-diabetic rats and genetically obese (fa/fa) rats using Northern blotting techniques. In both animal models one single transcript with a mean size of 4.12 kb could be detected. In insulin-deficient rats the Ca(2+)-ATPase transcript level decreased by 51% when compared to normal rats. In obese animals a modest decrease in the Ca(2+)-ATPase mRNA content to 77% of lean controls has been detected. Decreased mRNA expression of the SR Ca(2+)-ATPase may partly explain the delayed diastolic relaxation observed in the diabetic heart.