Early undernutrition of male and female rats was effected by limiting the mothers' food supply by 40% during pregnancy and lactation. The rats were killed at the age of 20 weeks and physiologically fast and slow skeletal muscles were prepared for histochemical demonstration of muscle fibre types. In the fast extensor digitorum longus (EDL) and the slow soleus (SOL) significant decreases in muscle weight were seen only in male experimental animals. Decreases in muscle fibre type cross-sectional area were found in the EDL in both males and females. In SOL, both fibre types showed evidence of fibre hypertrophy, and this was statistically significant in male animals. No changes were seen in the proportions present of the fibre types in either of the muscles.