In rats bled to invariably fatal haemorrhagic shock (mean arterial pressure = 18-24 mmHg), the prompt and sustained improvement of cardiovascular function, obtained with the i.v. injection of cholecystokinin octapeptide (CCK-8, 20 g/kg) is associated with a massive increase in the volume of residual circulating blood (0.69 +/- 0.12 ml/100 g b.w. in saline-treated rats; 1.61 +/- 0.09 ml/100 g b.w. in CCK-8-treated rats). The number of red cells/mm3 and the % Hb content is the same in CCK-8-treated and in control rats. So, in a condition of severe haemorrhage, otherwise incompatible with survival, the i.v. injection of CCK-8 not only induces an impressive increase in arterial pressure and in circulating blood volume, but also greatly improves tissue oxygenation.