The specific activities of the succinate dehydrogenase-coenzyme Q reductase were determined in mitochondria from the thymus and the spleen of aged mice (20, 22 and 24 months) as compared with young mice (10 weeks). Significant steep escalation of the deficiency of coenzyme Q-enzyme activity was observed in the thymus of all three groups of aged mice. No significant deficiency was found in the mitochondria of the spleen. The ratios between the liver weight:body weight and the spleen weight:body weight in young and aged mice are practically unchanged, but the thymus weight:body weight ratio decreases significantly in all three groups of aged mice. The described age-dependent anatomical and functional alterations in the thymus most likely form the base for the development of the T cell determined suppression of the immunological responsiveness, present in aged mice.