Elevation of blood pressure induced by an acute sodium and fluid load in the anephric state has been attributed to intravascular fluid volume expansion. The present experiments were designed to study the role of vasopressin and catecholamines in this type of hypertension. Normotensive anephric rats, adrenergically intact or pretreated with alpha- and beta-adrenoceptor blockade, and deoxycorticosterone (DOC)-salt-treated anephric rats, intact or pretreated with alpha- and beta-adrenoceptor blockade, received an infusion of 2 ml containing 3 meq NaCl, followed by intravenous administration of an analogue antagonist of the vasopressor effect of arginine-vasopressin (AVP). Pressure increments induced by hypertonic saline were abolished by an AVP antagonist partly in the adrenergically intact animals (leaving a small residual pressure elevation) and completely in adrenergically blocked animals, which had a larger AVP component. Volumes expansion did not necessarily accompany increase in blood pressure after saline infusion. In fact some DOC-salt-treated animals with the highest blood pressures and norepinephrine levels exhibited contraction of plasma volume. Increments in blood pressure were negatively correlated with plasma volume changes (r = -0.687, P less than 0.05) in these animals and positively with norepinephrine levels in all adrenergically intact animals (r = 0.818, P less than 0.001). It is concluded that the hypertensive response elicited by acute hypertonic saline load is due to vasoconstriction mediated partly by vasopressin and partly by the sympathetic system, which may in some way attenuate the effect of vasopressin.