The ontogenic changes in thyroxine 5′-monodeiodinase (5′-MD) activity were examined in brown adipose tissue (BAT) of fetal (26, 28 and 30 days of gestation) and newborn (1–14 days old) rabbits. A near-maximum stimulation of BAT 5′-MD activity was reached at 1 mM of dithiothreitol (DTT). The kinetics of 5′-MD inhibition by propylthiouracil (PTU) was uncompetitive in fetal and noncompetitive in neonatal tissue. In fetal tissue the K_i was 3.2 mM independently of DTT concentration, in newborns from 0.5 to 8.25 mM at 1–10 mM of DTT. An apparent K_m constant for T₄ to T₃ conversion in BAT of fetal rabbit amounted to 0.2, 0.67, 1.9 and 2.0 μM in the presence of 1, 2, 5 and 10 mM DTT, respectively. In neonatal rabbits Km was constant (0.72 μM) in various DTT concentrations. The presence of 5′-MDI and 5′-MDII in BAT of fetal and neonatal rabbit was identified. Using iopanoic acid and amiodarone as inhibitors, a decrease in the 5′-MDII:5′-MDI ratio from 8.9 to 2.7 after birth was observed. Changes in 5′-MD activity with advancing age were not correlated with the SH group content in the BAT.