Stop-flow studies were used to characterize solute uptake in isolated rat lungs. These lungs were perfused at 8 or 34 ml/min for 10-28 s with solutions containing ¹²⁵I-albumin and two or more of the following diffusible indicators: [³H]mannitol, [¹⁴C]urea, ³HOH, ²⁰¹Tl⁺, or ⁸⁶Rb⁺. After this loading period, flow was stopped for 10-300 s and then resumed to flush out the perfusate that remained in the pulmonary vasculature during the stop interval. Concentrations of ²⁰¹Tl⁺ and ⁸⁶Rb⁺ in the venous outflow decreased after the stop interval, indicating uptake from exchange vessels during the stop interval. The amount of these K⁺ analogs lost from the circulation during the stop interval was greater when the intervals were longer. However, losses of ²⁰¹Tl⁺ at 90 s approached those at 300 s. Because extraction continued after the vasculature had been flushed, vascular levels had presumably fallen to negligible levels during the stop interval. By 90 s of stop flow the vascular volume that was cleared of ²⁰¹Tl⁺ averaged 0.657 ± 0.034 (SE) ml in the experiments perfused at 8 ml/min and 0.629 ± 0.108 ml in those perfused at 34 ml/min. Increases in perfusate K⁺ decreased the cleared volumes of ²⁰¹Tl⁺ and ⁸⁶Rb⁺. Uptake of [³H]mannitol, [¹⁴C]urea, and ³HOH during the stop intervals was observed only when the lungs were loaded at high flow for short intervals. Decreases in ²⁰¹Tl⁺ and ⁸⁶Rb⁺ concentrations in the pulmonary outflow can be used to identify the fraction of the collected samples that were within exchange vessels of the lung during the stop interval and may help determine the distribution of solute and water exchange along the pulmonary vasculature.