The postnatal differentiation and growth patterns of the different histochemical muscle fibre types were studied at frequent age intervals in the extensor digitorum longus (EDL) muscle of normal rats and animals subjected to pre- and postnatal protein-calorie undernutrition. Three stepwise changes were seen in the fibre type composition of the EDL muscle during maturation. In newborn normal rats 2 histochemically and morphometrically distinct fibre populations occurred: small foetal type (type F) fibres or myotubes (about 90%), with the staining characteristics of the so-called type 2C fibres, and larger type 1 fibres (10%). At 5 days of age the percentage of type F fibres had dropped to 50 simultaneously with the emergence of a new population (40%) of type 2B fibres, intermediate in cross-sectional area between the remaining type F fibres and type 1 fibres. Between 15 and 20 days the small type F fibres practically disappeared with the emergence of a corresponding percentage (40%) of type 2A fibres, smaller than both type 1 and type 2B fibres; Between 20 and 60 days the percentage of type 1 rose from 10 to almost 30, type 2B fibres correspondingly diminishing in number but growing at a faster rate than either type 1 or type 2A fibres. In the undernourished rats the histochemical differentiation was retarded at birth, all fibres or myotubes being of the foetal type; However, large type 1 and type 2B fibres were seen at the age of 5 days and the histochemical maturation proceeded almost normally at later stages. On the other hand, at all ages there was a proportional reduction in the cross-sectional area of all fibre types, amounting to about 50% at 180 dyas, and not fully restituted by nutritional rehabilitation. These observations suggest that type F (type 2C) fibres are the undifferentiated precursors of all mature muscle fibres, their stepwise histochemical transformation raising the question of maturational differences in the different types of motoneurones.