In the masseter muscle, male and female types of isozyme distribution of lactate dehydrogenase (LDH) were clearly detectable by the 40th postnatal day: the percentage of muscletype LDH isozyme (LDH 5) was statistically higher in the male muscle than in the female one. No sexual difference was found in the distribution in the tongue. In the testectomized mice, the distribution pattern of LDH isozymes was shifted to the female type in the masseter muscle, but testectomy had no effect on the tongue. On the other hand, ovariectomy never affected the distribution patterns of LDH isozymes in either masseter muscle or tongue offemale mice. These findings suggest that androgenic hormones are involved in the appearance ofthe sexual dimorphism ofLDH isozyme distributions in the masseter muscle of developing mice. On the contrary, estrogenic hormones may not be involved in this appearance.

Differences in postnatal growth between male and female animals, and appearance of sexual dimorphism in various organs have been the subject of extensive studies. Studies on androgenic hormone effects on striated muscles were initiated long ago (23, 29). These early studies indicated that androgenic hormones had pronounced effects on the growth of the striated perineal musculature in the rat, and induced a strong sexual dimorphism. Moreover, Wainman and Shipounoff (29) reported that testectomy in rats caused a marked decrease in the volume of the perineal musculature, and administration of testosterone propionate prevented the effects of testectomy on the musculature. These remarkable changes were not detectable in other striated muscle in rats (l7), though the treatment had noteworthily caused a severe decrease. in the mass of temporal muscle in guinea pigs (l6). Gutmann er al.(ll) classified the temporal muscle in guinea pigs as ‘fast white’in the mature male and ‘fast red’in the mature female, and reported that