The s.c. injection of aldosterone (10 micrograms/kg) induces a release of vasopressin. The peak of plasma vasopressin level occurs at the same time as the late in vivo effect of aldosterone on passive 22Na efflux from arterial smooth muscle. These results indicate that vasopressin mediates the delayed in vivo effects of aldosterone on ouabain-insensitive 22Na efflux, since on the other hand, it has been possible to show that the action of the peptide is accelerated by a previous exposure to the mineralocorticoid. Indeed, after a 120-min pretreatment with 10(-8) M aldosterone, vasopressin induces an effect on 22Na efflux in 30 min, as opposed to the 120 min needed in the absence of the steroid.