Vasopressin (antidiuretic hormone) is emerging as a potentially major advance in the treatment of a variety of shock states. Increasing interest in the clinical use of vasopressin has resulted from the recognition of its importance in the endogenous response to shock and from advances in understanding of its mechanism of action. From animal models of shock, vasopressin has been shown to produce greater blood flow diversion from non-vital to vital organ beds (particularly the brain) than does adrenaline. Although vasopressin has similar direct actions to the catecholamines, it may uniquely also inhibit some of the pathologic vasodilator processes that occur in shock states. There is current interest in the use of vasopressin in the treatment of shock due to ventricular fibrillation, hypovolaemia, sepsis and cardiopulmonary bypass. This article reviews the physiology and pharmacology of vasopressin and all of the relevant animal and human clinical literature on its use in the treatment of shock following a MEDLINE (1966-2000) search.