The method of posthaemorrhagic blood volume (BV) determination by simple haematocrit measurement has been compared with the conventional isotope dilution technique. 51Cr tagged erythrocytes and 125IHSA were used to estimate RBC volume and plasma volume in non-starved male Sprague-Dawley rats. Two series of experiments were carried out by two different investigatory groups. Haemorrhage was inflicted by 60 or 90 min of haemorrhagic hypotension at 70 mm Hg, causing 41% and 56% loss of the initial estimated BVs, respectively. There was agreement in both series for the initial blood volume indices; RBC volume, 2.82 ml x 100 g-1 b.wt.; plasma volume 3.33 ml x 100 g-1 b.wt. and F cells, 0.91. Using the RBC volume data, the calculated residual BVs after haemorrhage corresponded accurately to the isotope measurements in both series. It is concluded that non-splenectomized rats may be used for accurate BV analysis after haemorrhage if the basal data for the strain used are known.