The Clinical Study Center is a 9-bed multicategorical unit designed to provide optimum environment for controlled, critical and extensive clinical investigation in man in many disciplines. The following are the most active research studies currently being conducted within the Center: Aldosterone production and plasma renin activity and concentration measurements in hypertension and their use in identifying adrenal adenoma versus hyperplasia; the secretion, metabolism, control and pathophysiologic significance of deoxycorticosterone; the mineralocorticoid hormones and renin in hypo- and hyperkalemic states. Energy metabolism, body composition, albumin pool size and distribution in uremic children and children on dialysis; study of the hyperoxaluria in small bowel disease in man; study of dose-dependent kinetics of diphenylhydantoin to determine efficacy at different therapeutic levels. The effect of phenobarbital on disposal of thyroxine in Graves disease; selective renal vein catheterization for plasma renin activity levels in patients with unilateral renal disease; bile salt metabolism in cholestasis, TSH reserve during and following suppressive thyroid therapy, perpheral thyroid hormone in Graves disease, hepatocellular disease and drug disposition.