The prophylactic and therapeutic effects of hydrocortisone (50 mg/kg) in severe haemorrhagic shock were evaluated by metabolic, biochemical and haematological investigations in a rabbit model. It was found that administration of hydrocortisone prior to severe haemorrhage had no beneficial effect on any of the values measured. Owing to haemoconcentration and marked mobilization of leucocytes it would appear that in pretreated animals the magnitude of the hypoxia was increased and led to greater tissue damage and higher levels of lysosomal enzymes than in rabbits which had not received pretreatment with hydrocortisone. On the other hand, hydrocortisone therapy combined with volume replacement 1 hour after the haemorrhagic insult had several beneficial effects. The metabolic capacity of liver and kidney tissues was improved, the lysosomal concentration remained within normal limits, and the mean blood pressure and pulse pressure were maintained better than in controls. However, it would appear that this beneficial effect is only exerted on tissue still in a reversible state of shock. There is therefore no beneficial effect on lung tissue metabolism, the lungs being more sensitive to hypoxic damage than either liver or kidney tissue. Administration of hydrocortisone results in the immediate release of endotoxins into the circulation. This might be due to its vasodilatory action on the microcirculation of the intestinal viscera.