Male Sprague-Dawley rats (SD) were kept in medium between the postnatal 80(th) and 130(th) day while environmental temperature (from 20 to 5 degreesC) and photoperiod were decreased gradually. The weight of the animals adapted to the cold and the weight and length of the external digitorum longus (EDL) muscle were measured. When compared with controls, in a cold environment the weight and fiber length of the muscle was reduced significantly. In addition, EDIL muscle was separated into two (red and white) by using the succinic dehydrogenase (SDH) method and the percentage of fiber types and fiber diameters according to mitochondria contents were measured. Transformations in muscle fibers (from Type IIb to Type IIa), decreases in fiber diameters (particularly Type IIb) and atrophic fibers (in Type IIa and Type IIb) were observed. However, normal values were obtained when the temperature returned to normal.