Levels of adrenal arterial [K⁺] were varied by close intra-arterial infusion of [K⁺] enriched solutions to the single autotransplanted adrenal gland of the conscious sheep. Secretion rates of aldosterone, corticosterone and cortisol were determined before and during infusion of KC1 solutions. In both Na replete and Na depleted sheep, elevation of adrenal arterial [K⁺] was associated with increased aldosterone secretion, even with small (0.5 mEq/1 or less) changes in plasma [K⁺]. Larger elevations of plasma [K⁺]—up to about 1 mEq/1—produced correspondingly larger elevations in aldosterone secretion; increases beyond 1 mEq/1 produced little further effect. A similar order of responsiveness to similar elevations of plasma [K⁺] was seen in Na replete and Na depleted sheep. (Endocrinology85: 381, 1969)