Total blood volume and relative blood volumes in selected tissues were determined in non-anesthetized, confined rainbow trout by using 51Cr-labelled trout erythrocytes as a vascular space marker. 2. Mean total blood volume was estimated to be 4.09 +/- 0.55 ml/100 g, or about 75% of that estimated with the commonly used plasma space marker Evans blue dye. 3. Relative tissue blood volumes were greatest in highly perfused tissues such as kidney, gills, brain and liver and least in mosaic muscle. 4. Estimates of tissue vascular spaces, made using radiolabelled erythrocytes, were only 25-50% of those based on plasma space markers. 5. The consistently smaller vascular volumes obtained with labelled erythrocytes could be explained by assuming that commonly used plasma space markers diffuse from the vascular compartment.
