The ultrastructural changes seen in muscle fibres of aged rats have been studied and described in relation to the degree of fibre atrophy and to the involvement of the different fibre types. The ultrastructural abnormalities were classified into four different categories: (1) diffuse dilatation of sarcoplasmic reticulum, (2) streaming of Z-bands, (3) so-called myofibrillar degeneration, and (4) myofibrillar "homogenization". These are all, except (1), abnormalities which affect principally the contractile elements of the muscle fibre. The ultrastructural changes represent a chronic process. The abnormality most often encountered and hence the most important is myofibrillar degeneration. Of these four types of change, myofibrillar degeneration and myofibrillar "homogenization" may be degenerative in nature and lead eventually to the total dissolution of the muscle fibres involved. Smudging of Z-bands and diffuse dilatation of the sarcoplasmic reticulum, although occasionally diffuse and massive, were often found in otherwise normal muscle fibres and were rarely observed in severely atrophic ones. We therefore conclude that these latter two types of change are probably reactive in nature and are not ultimately lethal to the muscle fibres involved. Longitudinal sections of affected muscle fibres clearly showed that the morbid process involves a muscle fibre unevenly and not simultaneously over its entire length. Various stages of myofibrillar degeneration, for example, could be observed at the same time in different portions of one and the same muscle fibre. Streaming of Z-bands was observed exclusively in what appeared to be red fibres, while myofibrillar degeneration was observed predominantly in white or intermediate fibres. However some of these muscle fibres with myofibrillar degeneration could have been red fibres which had lost their mitochondrial population. Myofibrillar "homogenization" occurred in both red and white fibres. Other miscellaneous changes within muscle fibres are briefly described and discussed. Although the findings observed are in general non-specific in nature, the importance of neurogenic mechanisms is inferred. The occurrence of age-related ultrastructural alterations may have important implications in the study of muscle in general.