The periodic acid-Schiff (PAS) reaction and tests for glycogen phosphorylase, oxidative enzymes and acid-stable and alkali-stable adenosine triphophatase (ATPase) were used to determine the degree of histochemical differentiation between myofibres of sartorius muscles from neonatal piglets. Within 24 h of birth, the ratio of myofibres with acid-stable ATPase to those with acid-labile ATPase was 1:21-0. By 10 days the ratio had changed to 1:5-2. Mean minimum myofibre diameters (all histochemical types combined) increased steadily after birth although diameters of myofibres with acid-stable ATPase showed no increase until 6 days. By 10 days, consistent differentiation was observed with the alkali-stable ATPase and glycogen phosphorylase reactions but not with the PAS reaction. During the 10 day neonatal period, all types of myofibres contained large or moderate numbers of mitochondria.