The effects of aging on motor units of different types and the spatial arrangement of motor units are discussed. This article is, however, not a comprehensive survey of the literature, but rather a summary of our own results with the addition of relevant reports. The glycogen depletion technique was used to identify the muscle fibers in a motor unit (motor unit fibers). The motor unit fibers were subsequently characterized according to immunocytochemical, enzyme-histochemical and morphometrical properties, and the spatial arrangement was determined with the aid of a computer-assisted model. An age-related motor unit transformation was observed in both fast- and slow-twitch rat skeletal muscles as was a rearrangement of motor unit fibers. The spatial redistribution of motor unit fibers, higher innervation ratios, and larger motor unit territories in old age indicate an age-related denervation-reinnervation process due to loss of alpha-motoneurones.