In this study the differentiation of postnatal muscle fibres in dog se
mitendinosus muscle has been characterized. Several histochemical tech
niques for myosin ATPase and metabolic activity were used in animals a
ged between 1 day and 2 months. The results show that at birth there a
re two types of fibre, whose ATPase activity gradually changes during
postnatal development, so that several types of fibre can be identifie
d after 2 months. These finally become the four types of the adult: I,
IIA, IIDog, IIC. The main conclusions of the study are that the use o
f mATPase techniques is appropriate for showing the differentiation be
tween muscle fibres, even at early stages of postnatal development, an
d that the origin of the four different fibres of the adult can be tra
ced back to early postnatal stages.