Histomorphological and histomorphometrical observations were used to describe the development of masticatory muscles from normal and muscular dystrophic mice. The masseter and the digastric muscle were described from the birth to 35-40 week of age. It has not been possible by histomorphological criteria to separate dystrophic muscles from normal muscles at birth. From 2 weeks onwards marked differences between the affected and unaffected muscles appeared, as the affected fibres from this age are rounded with marked variations in size. Central nucleation is frequent and there is an increased amount of connective tissue between the fibres. The histomorphometrical observations revealed an increase in mean size of the fibres with age, both in normal and dystrophic masticatory muscles. The fibre size variance which has been shown to be a reliable parameter for description of degree of affection of dystrophic muscles, increased with increasing age in both groups. However, the variance is at all ages greater in the dystrophic muscles than in the normal ones, and is always greater in the masseter muscle than in the digastric muscle. There seems to be some small differences between male and female masticatory muscles, whereas no differences could be revealed between muscles from normal and heterozygous animals. Possible explanations of the obvious differences in degree and progression of the disease between the masseter and digastric muscle are proposed.