In six circuit experiments using a clinical hemofiltration device, we validated a colorimetric technique to measure transvascular volume exchange (VE). In 12 isolated excised canine left lower lobes, continuous colorimetric measurements of VE correlated well with calculations of VE from changes in microhematocrit obtained simultaneously. We introduced step increases in microvascular hydrostatic pressure (Pc) of 9 +/- 4.8 (SD) cmH2O and followed the time course of weight and continuous hematocrit changes measured colorimetrically for 40 min, after which Pc was returned to base line, while measurements were continuously obtained. This procedure was repeated for an additional 30 min. VE was calculated from the hematocrit signals and compared with the time course of the weight signal. After increases in Pc, followed by a rapid weight gain, weight signals followed a slow exponential time course, whereas the calculated VE changed linearly. VE reflected approximately 60% of the slow weight gain. When Pc was decreased, weight signals decreased exponentially, whereas VE continued to increase linearly at a slower rate. These results suggest that a significant component of the slow weight signal represents slow vascular volume changes. Contrary to what the weight signal suggested, edema was never reabsorbed over the range of Pc measured.