1. The activity of acid phosphatase (AP) were studied both bio- and histochemically in fast and slow muscles of the rat during postnatal development. 2. Both biochemical and histochemical methods show at birth a similar, high AP activity in both, fast and slow muscles. From 20 days onward AP activity is higher in the slow soleus than in the fast extensor digitorum longus (EDL) muscle. 3. Fibres with high AP activity have also higher activities of oxidative enzymes and of non-specific esterase (NSE). This property is not coupled to degree of ATP-ase activity; fibres with high or low ATP-ase activity reveal similar activities with respect to AP. 4. There is a "two-phase" developmental change in the degree of enzyme activity in both the fast EDL and slow soleus muscles. AP activity is high at birth, decreases during later postnatal development and increases again in senescent muscles.