The excretion of Tl+-ions from blood into the lumen was investigated on the stomach and on segments of jejunum, ileum, colon ascendens, and colon descendens in situ of anesthetized rats using the pendular perfusion technique. The kidneys of the rats were tied-off. Tl+-ions were administered i.v. at the beginning of the experiments as 204Tl-(Tl2SO4). The amount of 204Tl-labeled Tl+-ions in the perfusion fluid of the gastrointestinal sections was calculated from the 204Tl concentration and the fluid volume. 2. Excretory activity for Tl+-ions is highest in the the jejunum; it is followed by the colon ascendens. The ileum and the colon descendens show nearly equal excretory activity which is just half of that measured in the jejunum. The amount of Tl+-ions excreted into the stomach is negligible. 3. At the end of the experiment in the perfusion fluid of all segments of the intestine the concentration of 204Tl is statistically significantly higher than that in plasma. 4. In jejunal and colonic (descendens) segments the amount of Tl+-ions excreted increases with increasing i.v. doses tested from 2 x 10(-8) up to 2 x 10(-6) moles/kg body weight.
