To determine the effect of CRF administration on in vivo mineralocorticoid release, we performed experiments in rats and in man. In man, plasma aldosterone and 18-hydroxycorticosterone levels increased following i.v. CRF injection. In sitting subjects, the response was greater than in supine subjects, indicating an influence of posture. In the rat, CRF-induced increases in corticosteroid levels were dose-related, the largest response being for corticosterone, followed by 18-hydroxycorticosterone and then by aldosterone. Thus synthetic ovine CRF stimulates not only glucocorticoid but also mineralocorticoid secretion from the adrenals.