Vasopressin has been shown to elicit vasoconstriction in unanaesthetized animals at plasma concentrations similar to those associated with its renal antidiuretic effect. The vasconstrictor effects of vasopressin do not normally translate into pressor responses until relatively high plasma concentrations are reached. This appears to be related to very effective buffering by the baroreceptor reflex. In the absence of afferent signals from the baroreceptors (surgical denervation, but more importantly, low arterial pressure), the vasoconstriction elicited by vasopressin represents a significant part of the mechanisms that determine blood pressure. Vasopressin is clearly involved in the short-term control of blood pressure in situations such as haemorrhage, other volume-depleted states and dehydration. However, it is only one of several short-acting mechanisms which complement each other in the defence against hypotensive stresses. Under different conditions, the cardiovascular effects of vasopressin seem to have a component related to the central nervous system control of the circulation. Whether or not circulating vasopressin interacts with the newly described network of extrahypothalamic projections from the paraventricular nucleus is yet conjectural. However, the presence in the brain of vasopressin-containing pathways and of various types of receptors to vasopressin, as well as the existence of cardiovascular effects elicited by central administration of antidiuretic hormone, suggests a role for cerebral vasopressin in the control of autonomic function. Slightly elevated levels of vasopressin have been found in various forms of hypertension. Yet, the role of vasopressin, when present, may be more related to its antidiuretic than to its vasoconstrictor properties.(ABSTRACT TRUNCATED AT 250 WORDS)