Circulating thyroxine (T4), triiodothyronine (T3) and reverse triiodothyronine (rT3) as well as blood lactate and glucose concentrations were measured in a group of 12 trained volunteer subjects prior to and after swimming 0.18 or 0.9 km, to determine if increase in metabolic activity was accompanied by diversion of T4 monodeiodination from the active (T4 to T3) to the inactive (T4 to rT3) pathway. The resting T4, T3, and rT3 levels were 8.5 micrograms . 100 ml-1, 108 ng . 100 ml-1, and 57 ng . 100 ml-1, respectively, whereas after 0.18 km of swimming the corresponding levels were 9.5 micrograms . 100 ml-1, 135 ng. 100 ml-1 and 70 ng . 100 ml-1. After 0.9 km of swimming, T4, T3, and rT3 levels were 9.0 micrograms . 100 ml-1, 126 ng . 100 ml-1, and 66 ng . 100 ml-1, respectively. The swimming was accompanied by hemoconcentration and increase in blood lactate but not in glucose concentrations. In two other investigations thyroid hormones were measured prior to and after 60 or 90 min of moderate exercise on a bicycle ergometer. This exercise had no effect on circulating thyroid hormone levels. Free thyroxine (FT4) concentration and thyroxine binding globulin (TBG) capacity were unaltered after exercise. In conclusion, brief strenuous swimming or moderate bicycle exercise had minor or no effect on thyroid hormone concentrations when consideration was given to the attendant hemoconcentration. Even when exercise induced small T3 and rT3 changes were noted, they were in the same direction (increase) thus demonstrating a lack of diversion of peripheral T4 monodeiodination.