We analyze the effect of chronic undernourishment on extensor digitorum longus (EDL) muscle maturation in the rat. Cytochrome c oxidase (COX) and alkaline ATPase histoenzymatic techniques were used to determine the relative proportion of different fiber types (oxidative/glycolytic and type I, IIa/IId, or IIb, respectively) and their cross-sectional area in control and undernourished EDL muscles at several postnatal (PN) ages. From PN days 15 to 45, undernourished EDL muscles showed predominance of oxidative and type IIa/IId fibers, but from PN days 60 to 90, there were a larger proportion of oxidative fibers and an equal proportion of type IIa/IId and IIb fibers. Meanwhile, in adult stages (from PN days 130-365), the relative proportion of fiber types in control and undernourished EDL muscles showed no significant differences. In addition, from PN days 15 to 90, there was a significant reduction in the cross-sectional area of all fibers (slow: 13-53%; intermediate: 24-74%; fast: 9-80%) but no differences from PN days 130 to 365. It is suggested that chronic undernourishment affects the maturation of fast-type muscle fibers only at juvenile stages (from PN days 15-45) and the probable occurrence of adaptive mechanisms in muscle fibers, allowing adult rats to counterbalance the alterations provoked by chronic food deprivation.