We studied the regulation of type II 5'-deiodinase (5'D-II) activity and uncoupling protein (UCP) messenger RNA (mRNA) by thyroid hormones in fetal brown adipose tissue (BAT). Fetuses were obtained from hypothyroid pregnant rats infused with increasing doses of T4 or T3. Infusion of T4 into hypothyroid pregnant rats increased T4 and T3 concentrations and inhibited 5'D-II activity in fetal BAT. In contrast, infusion of T3 increased BAT 5'D-II activities at low, normal, or high BAT T3 concentrations. The relationship between thyroid hormone concentrations in fetal BAT and plasma showed that BAT T3 concentrations are relatively stable, increasing less than 2-fold over a wide range of circulating T4 (3-fold) or T3 (8-fold) concentrations. Most T3 in fetal BAT are locally derived from T4 and not from plasma T3. UCP mRNA expression decreased to 30% of control values in hypothyroid fetuses. UCP mRNA levels were restored to normal in parallel with BAT T3 concentrations after the infusion of either T4 or T3. UCP mRNA levels correlate well with BAT T3 concentrations. Supraphysiological doses of T4 did not further increase either BAT T3 or UCP mRNA levels. T3 might regulate basal UCP mRNA expression during fetal life.