Spironolactone bodies were observed in an adrenal cortical adenoma that was removed from a patient with primary hyperaldosteronism (Conn syndrome) treated preoperatively with spironolactone. The electron microscopical evaluation of this adrenal cortical adenoma shows origin of spironolactone bodies from whorls of endoplasmic reticulum in cells with the cytoplasmic features of those from the zona glomerulosa of the adrenal gland. There was no evidence that the bodies were derived from mitochondria, which confirms recent ultrastructural findings in patients treated with spironolactone. These bodies have been described in the adrenal cortex only in patients who have received spironolactone, and the pharmacologic specificity of the bodies strongly suggests a direct mode of action by spironolactone on aldosterone production by cells of the adrenal zona glomerulosa.