Previous studies have suggested that thyroid hormone influences maturation of the renin angiotensin system (RAS) and cardiovascular function in the late gestation fetal sheep. To further examine the importance of thyroid hormone in this regard, we utilized the technique of thyroidectomy (TX) to remove endogenous thyroid hormone from the circulation, and then replaced it with physiological amounts of exogenous thyroxin. We hypothesized that the previously observed changes in RAS activity and cardiovascular function associated with TX would be normalized. TX was performed at 120 days of gestational age (dGA), and control fetuses were sham operated. After three days recovery, TX fetuses were continuously i.v. infused with thyroxin until delivery by cesarean section close to term (around 138 dGA). Immediately prior to necropsy fetuses were infused with isoproterenol and the hemodynamic responses were noted. Thyroid hormone replacement normalized not only plasma T3 and T4 levels, but also the TX induced decreases in renal renin mRNA and renal renin content. Renal angiotensin II subtype receptor expression levels were also normalized at both the mRNA and protein level. Decreased basal heat rate and systolic blood pressure associated with TX returned to normal following replacement; however changes in mean blood pressure and isoproterenol induced changes in mean blood pressure were not altered. These findings demonstrate that replacement of thyroid hormone in hypothyroid sheep fetuses can restore renal angiotensin II receptor and renin expression and secretion to normal.