Yearling red deer (Cervus elaphus) stags where either treated with 1 mg triiodothyronine (T₃) weekly from 18 January to 28 March then 5 mg (T₃) weekly from 5 April to 24 May 1990 (n=4), or served as controls (n=4). From 18 January to 14 June weekly blood samples and measurements of live weight, testicular diameter and heck girth were taken. Plasma testosterone and LH responses to a GnRH challenge were determined each month and plasma T₃ concentration was measured in the weekly blood samples. The T₃ treatment produced highly significant elevations of T₃ in plasma. However no effect of increased plasma T₃ was seen in any of the other parameters. These results indicate that the thyroid hormones are not involved in regulation of autumnal transition to the breeding season in red deer stags.