Studies were carried out to demonstrate the effect of sodium pentobarbital and morphine-pentobarbital anaesthesia on the haemodynamic and sympathoadrenal responses of dogs in haemorrhagic shock. The results were compared to those of conscious dogs and of dogs receiving only morphine. The reactivity of the cardiovascular system to exogenous adrenaline was also studied in every group. Pentobarbital did not change the resting plasma level of catecholamines but reduced the increase of plasma catecholamines in shock. In the pentobarbital-anaesthetized dogs in which shock was less severe, the reactivity of blood vessels to adrenaline was higher than in conscious dogs. In the haemodynamic response to shock the increase of heart rate predominated in the conscious dogs while the rise of blood pressure was more pronounced in the pentobarbital-anaesthetized animals. Subcutaneously injected morphine decreased the heart rate and increased the plasma catecholamine and histamine levels. Morphine-pentobarbital anaesthesia decreased the resting arterial pressure and increased the plasma histamine level while the plasma catecholamines were near their initial levels. Morphine alone increased the plasma catecholamines, and the subsequent shock could not induce as high a sympathoadrenal response as in the conscious dogs. Pentobarbital administered following morphine decreased the plasma level of catecholamines nearly to the conscious level but could not inhibit the sympathoadrenal activation induced by shock. Anaesthesia is a significant additional factor in the evaluation of shock experiments carried out on anaesthetized animals.