Total, packed cell and, plasma volume estimates were made for the whole body and selected tissues of rainbow trout by the simultaneous injection of radiolabelled trout erythrocyte (⁵¹Cr-RBC) and radioiodinated bovine serum albumin (¹²⁵I-BSA) tracers. Blood volumes were estimated with both markers separately by the tracer-hematocrit method and as the combination of the ⁵¹Cr-RBC packed cell and ¹²⁵I-BSA plasma volumes. Mean whole body blood volume was significantly less when calculated from the ⁵¹Cr-RBC tracer data (3.52±0.78 ml/100 g; ±SD) than when calculated with the ¹²⁵I-BSA tracer (5.06±0.86 ml/100 g) or as the sum of the two volumes combined (4.49±0.60 ml/100 g). The whole body hematocrit (28±5%), estimated as the quotient of the ⁵¹Cr-RBC volume divided by the sum of the ¹²⁵I-BSA and the ⁵¹Cr-RBC volumes, also was significantly less than the dorsal aortic microhematocrit (36±4%). Estimates of total blood volumes in most tissues were significantly smaller when calculated from the⁵¹Cr-RBC data than when calculated by the other two methods. Tissue blood volumes were greatest in highly vascularized and well perfused tissues and least in poorly vascularized tissues. The relative degree of vascularization among tissues generally remained the same regardless of whether the red cell or the plasma tracer was used to calculated blood volume. It is not clear whether the expanded plasma volume is the result of the distribution of erythrocyte-poor blood into the secondary circulation or the result of extravascular exchange of plasma proteins.