Spontaneous autoimmune thyroiditis (SAT) is detectable by 2 weeks of age in the obese (OS) strain of chicken, and by 6 weeks virtually all of these animals have severe lymphoid infiltration of the thyroid. During the interval from hatching to 6 weeks of age, testosterone implants were administered for varying periods. All treatment groups were terminated at 6 weeks. Testosterone administered shortly after hatching increased thymus weight, depressed bursa weight, and reduced the severity of SAT, as measured by lymphoid infiltration of the thyroid and circulating thyroglobulin autoantibody concentrations. Birds supplemented with testosterone at later intervals exhibited depressed bursa and thymus weight; however, the severity of SAT was not affected. In a second experiment, the treated group was administered testosterone for the first 2 weeks after hatching. Thyroids were histologically examined at 2 and 6 weeks using a panel of five monoclonal antibodies (anti-Ia, anti-IgM, anti-IgG, anti-T cell, and antithymocyte). Compared with untreated controls, testosterone treatment reduced the frequency of Ia+ cells, T cells, and IgG+ cells in the thyroid. This treatment did not affect the frequency of IgM+ cells, and perhaps slightly increased the frequency of thymocytes. These results suggest that testosterone's suppressive effects are associated with changes in thymic development and effector T-cell and regulator T-cell activity.