Previous studies have shown that the thyroid must be important during the onset of photorefractoriness because thyroidectomised starlings (Sturnus vulgaris) do not become photorefractory. Therefore, to determine changes in plasma thyroxine during the onset of photorefractoriness, starlings were kept on photoperiods which induce photorefractoriness at different rates. Four groups of males and females were transferred from 8 hr light:16 hr darkness (8L:16D) to either 18L:6D, 13L:11D, 11L:13D, or 8L:16D and weekly blood samples were taken and assayed for thyroxine. In males and females on the two longer photoperiods, plasma thyroxine increased to a peak after 2-3 weeks. In those on 18L:6D, this was followed by a second peak after 9 weeks. Both 18L:6D and 13L:11D induced photorefractoriness. In birds held on 11L:13D, which became sexually mature but did not become photorefractory, there was no change in thyroxine. The results demonstrate that an increase in thyroxine concentration is associated with the onset of photorefractoriness.