Severe haemorrhagic shock was studied in a series of animal experiments. Reduction in myocardial oxygen and substrate utilisation in association with cardiac failure was demonstrated when the duration of shock was prolonged. The probable aetiological factors included ischaemia and the presence in the blood of certain vasoactive and metabolic depressant substances. It was concluded that the process leading to irreversible shock was multi-factorial in origin but failure appeared to play an important role.