In this study, we examined the effect of vanadate treatment on cardiac changes recognized in diabetic rats. In addition, the possible contribution of thyroid hormones to vanadate's effect on alloxan-diabetic atria was also investigated. Administration of alloxan to rats, as expected, resulted in hyperglycemia; hypoinsulinemia reduced thyroid hormone levels, decreased body weight and depressed cardiac function. Vanadate treatment of diabetic rats normalized blood glucose and serum thyroid hormone levels, neither were serum insulin levels of diabetic animals corrected after vanadate treatment. Vanadate treatment, however, did not affect the body weights of diabetic rats. Spontaneously-beating atria from diabetic rats were found to have decreased rates but increased forces of contractions compared with those from controls. On the other hand, the responsiveness of diabetic atria to both inotropic and chronotropic effects of isoprenaline was found to be decreased. Vanadate treatment resulted in the normalization of these alterations observed in diabetic atria. These results thus indicate that the normalizing effect of vanadate on diabetes-induced hypothyroidism may contribute to its effect in preventing cardiac changes observed at the early stage of diabetes.