An examination of the extent of reversibility of thyroxine binding within the thyroxine distribution space in the rabbit.

1. New Zealand white rabbits were injected intraperitoneally with [(125)]L-thyroxine. Experiments were performed after 24 hr had elapsed.2. Thyroxine distribution space was calculated as 129 ml. kg(-1). Half-life of disappearance of thyroxine from the blood of the urethane anaesthetized rabbit was 653 min.3. After exchange of approximately 20 ml. kg(-1) whole blood with heparinized blood from a non-radioactive donor, equilibration of radioactivity in the recipient had produced a peak radioactivity by 2 hr.4. The thyroxine space in which complete exchange took place within 2 hr was 102.7 +/- 4.7 ml. kg(-1) in six rabbits. This is 79.6% of the thyroxine distribution space.5. These results are compared with observations in the rat.