The metabolic effects of moderately severe gastrointestinal haemorrhage were investigated in man. Before resuscitation, patients had raised circulating concentrations of glucose, lactate, alanine, glycerol and cortisol. After urgent operation for haemorrhage, metabolite concentrations were similar to those of control patients having elective abdominal surgery, but insulin concentrations were higher and cortisol lower in haemorrhage patients. There were no significant differences in nitrogen excretion between haemorrhage patients and their controls, but urinary 3-methyl-histidine excretion by haemorrhage patients was lower indicating decreased muscle protein breakdown. Decreased amino acid release from muscle might account for previously reported imparied wound healing after haemorrhage.