OBJECTIVE Vasopressin is critical for blood pressure regulation when cardiovascular homeostasis is threatened and some patients with shock have inappropriately low levels of hormone in plasma. The present review focuses on recent work that addresses the role of endogenous vasopressin in the pathogenesis of shock and the potential therapeutic indications and secondary effects of exogenous hormone in patients with shock. RESULTS Examples of types of shock resistant to catecholamine pressors in which exogenous vasopressin was effective in restoring arterial pressure continued to accumulate. Widespread determinations of plasma vasopressin in patients with shock suggest that endogenous vasopressin deficiency may be more frequent than previously thought. The generation of mice with deletion of vasopressin's V1a receptor highlighted the important role of the hormone on cardiovascular homeostasis. CONCLUSIONS Vasopressin administration is very effective in restoring arterial pressure in many forms of shock and this appears to be due, at least in part, to deficiency of endogenous hormone. Generation of mice lacking vasopressin V1a receptor open new and exciting avenues of inquiry to clarify the role of the hormone in cardiovascular homeostasis.