The development of thyroid function in altricial ring doves was assessed by measuring thyroid hormone concentrations in the serum, hormone content of the thyroid glands and hepatic 5'-monodeiodinase activity. Thyroid function is low at hatching and increases during the first eight days while the nestlings are ectothermic and completely dependent on parental care. The rate of increase of serum hormone concentrations slows after Day 8; hormone concentrations are stable by Day 15 and for the remainder of the nestling and early fledgling periods while locomotor ability, feeding self-sufficiency and thermoregulatory ability are maturing. Increases in serum hormone concentrations precede increases in thyroidal hormone content. T3/T4 ratios in serum are much higher than those of stored hormones in the thyroid. Deiodination of T4 to T3 is important in T3 production throughout development but most so in the early nestling stages.