For the purpose of blood volume determination the mixing time and disappearance rate of R131IHSA and 51Cr-labelled erythrocytes was measured in rats under normal conditions and in hemorrhagic shock. Using a reservoir technique the mean arterial blood pressure of rats was reduced to 30 mm Hg over a period of 240 min (stage 1), 270 min (stage 2), and 300 min (stage 3). After this time the equilibration pattern and the blood volume were found to be markedly changed. The mixing time of both tracers was prolonged up to 6-fold in shock stage 1 and to 30-fold in shock stage 3. Whereas the disappearance rate of R131IHSA increased from 7 to 18%/h. Blood volume values measured with R131IHSA were always found to be higher than values determined with "Cr-erythrocytes. Depending on the shock stage blood volume was decreased with both methods. Only in shock stage 3 a reverse proportion of R131IHSA values to 51Cr-erythrocyte values was observed and could be explained by incomplete mixing of the tracer. These investigations show clearly that blood volume measurements are only reliable if changes in the mixing time and the disappearance rate are known.