Since 1957 clinical studies and animal experiments have reported that a solution of hydroxyethyl starch in saline (HES) was effective as a plasma expander. We have studied hydroxyethyl starch as compared to dextran after experimental bleeding. Thirteen dogs averaging 10 kg body weight were utilized in this study. Without premedicants anesthesia was induced with intravenous thiopenthal sodium and subsequently endotracheal intubation was performed. Respiration was controlled with a Harvard Pump, setting the respiratory rate at 15 times a minute and tidal volume at 20 ml/kg. Arterial blood pressure, central venous pressure and EKG lead II were recorded by polygraph. RIHSA was used for the measurement of plasma volume and extracellular volume. Experimental bleeding was equalled 25ml/kg for 15 minutes and the same amount of plasma expander (HES or Dextran) was infused 30 minutes after bleeding. The results were as follows: 1. Blood pressure was markedly decreased after experimental bleeding, but on administration of HES or Dextran it returned to nomal values. In the control group blood pressure persisted at about 90 mmHg. Central venous pressure after the administration of HES or Dextran also recovered to the level before bleeding. Pulse rate was slightly decreased after infusion of plasma expanders. 2. The values of hemoglobin and hematocrit were reduced about 40 percent after infusion of HES or Dextran. 3. Plasma volume was increased about 130 percent and blood volume was increased about 110 percent immediatly after infusion of HES or Dextran. A significant change in ECF could not be observed. 4. A persistent dilutional acidosis after infusion of HES or Dextran was not found. 5. In the HES or Dextran groups osmotic diuresis was observed. In the results of our experimental study no significant differences between HES and Dextran were observed, and it is thought that HES is safe and useful as a plasma expander.