During a four day period, each of 14 healthy mongrel dogs that had splenectomy was twice bled 450 milliliters of blood and twice reinfused with 500 milliliters of isotonic sodium chloride solution. On the sixth day, the dogs were hypovolemic and anemic; they had 10 per cent reductions in total blood volume and 50 per cent reductions in erythrocyte volume, but no change in plasma volume. Seven of the dogs were transfused with autologous washed erythrocyte concentrates and seven others with an equal volume of autologous plasma. Two units of autologous washed erythrocyte concentrations with hematocrit values of 80 volumes per cent increased plasma volumes to levels similar to those achieved with 2 units of autologous plasma. The serum oncotic pressure, total protein and albumin concentrations were similar in the two groups, except that, two hours after transfusion, the serum albumin level was significantly higher in the dogs transfused with plasma. These data demonstrate that erythrocyte transfusions increase both the erythrocyte and plasma volumes.