Serum thyroid function (T4, T3, rT3 and TSH levels) and measures of peripheral thyroid hormone action (serum dopamine-β-hydroxylase activity (DBH) and sex hormone binding globulin (SHBG)) were determined in 6 women before and one month after initiating an aerobic physical conditioning program. The same measurements were made in a control group of 6 women who did not increase their activity during this time. In physically conditioned subjects, the resting heart decreased from 65.1 ± 3.9 (mean ± SE) at baseline to 58.0 ± 2.9 beats per minute after one month (P < 0.025), indicating an appreciable state of physical conditioning was achieved. However, there were no statistically significant changes in thyroid function test, serum DBH or SHBG levels in either the physically conditioned or the control group. These data indicate that being physically conditioned has a negligible effect on thyroid status.