A comparison is made of relative organ-to-plasma T1-201 uptake in rats at rest and after exercise (2 h swimming). Organs were obtained 1/2 or 3 h after T1-201 injection. Under the four experimental conditions, the largest uptake was found in kidneys, heart and adrenal glands. The post-exercise to rest ratio of myocardial uptake is in agreement with clinical data reported in the literature. Two extensive tissues, skeletal muscle and fat, showed a remarkable increase of T1-201 uptake at 3 h as compared to 1/2 h in both rested and post-exercised animals. During the same time interval there was a temporal "washout" of T1-201 from heart (both at rest and after exercise) and kidney (after exercise only). The findings do not support the view of a delayed myocardial uptake phase of T1-201 during which T1-201 would be reintroduced into the blood from other body organs.