The effect of altered thyroid status on the development of beta-adrenergic receptor (BAR) density and affinity was investigated in ovine fetal and newborn heart and lung. Fetal (119-121 days gestation) and newborn (2-3 days of age) sheep underwent either thyroidectomy alone, thyroidectomy plus infusion of a large dose of T3 or sham operation. Eight days later BAR were measured in heart and lung using the tritiated radioligand dihydroalprenolol. There was no apparent effect of altered thyroid status on fetal heart or lung BAR density or affinity. In contrast, the newborns thyroidectomized and infused with T3 had a 96% increase in heart BAR density and an 83% increase in lung BAR compared to the thyroidectomized only or sham-operated newborns which were similar. These results suggest that in the near term ovine fetus heart and lung tissue BAR are neither dependent on nor responsive to thyroid hormones, whereas in the newborn period heart and lung BAR are highly responsive to thyroid hormones.