In the case reported, an overproduction of aldosterone was accompanied by paretic attacks, a decrease in the serum potassium level, and in muscle tone and the deep tendon reflexes. The decrease of serum potassium level was consistent, but moderate, being to just below the normal lower limit. Loading with carbohydrate produced an attack, forearm ischaemic exercise resulted in less lactic acid production than normally. In the limb-girdle muscles there was also evidence of slight but permanent weakness. The results of histological and ultrastructural examination of a muscle biopsy are reported. It seems probable that the disturbance in the mineralo-corticoid metabolism led periodically to a decrease in the serum potassium, while in some muscle fibres the glycogen content increased and the SR became dilated and partially destroyed. In the attack-free periods tubular aggregates were produced, supposedly through an attempt at regeneration of the SR. During attacks portions of the muscle fibres were destroyed irreversibly and in consequence moderate permanent weakness resulted. Some spheromembranous degradation products which were observed might be regarded as remnants of degenerated tubular aggregates.