The levels of thyroid hormones in the plasma and the activities of 5'-deiodinase activity in liver and kidney were determined in the tammar wallaby, Macropus eugenii, from early pouch life to adulthood. The total concentration of plasma thyroxine (T4) was below 15 nmol/l before day 75 of pouch life, rose to about 75 nmol/l at day 160, and then decreased to about 12 nmol/l in the adult. The total concentration of plasma tri-iodothyronine (T3) was below 0.4 nmol/l before day 120, increased to 3 nmol/l by about day 220 and then decreased to 1.0 nmol/l in adults. Concentrations of free T4 and free T3 followed a similar pattern but peaked at 45 and 160 pmol/l respectively. Concentrations of reverse T3 (rT3) were extremely variable, ranging from 0 to 1 nmol/l at day 100, and from 0 to greater than 2 nmol/l at day 180. After about day 230, rT3 levels fell rapidly and were below 0.3 nmol/l in adults. Liver and kidney 5'-deiodinase activities, which were undetectable before day 80, reached adult levels by day 220. Half-maximal activity of both these enzymes occurred at about day 205, mid-way between the peaks of T4 and T3. These findings suggest that the systems supporting synthesis and release of hormones from the thyroid gland are probably mature by about day 160 of pouch life in the tammar, while peripheral deiodinase activity, which is a major factor in the production of T3 in the plasma, matures by about day 220. These events thus precede the development of physiological independence of the young tammar from its mother.