Studies on pathophysiology in mouse trichinellosis have revealed a need for information on the temporal relationship between myopathophysiologic alterations and the development of myositis in trichinous mice. Most information presently available on trichinous myositis is qualitative, derived from histological studies carried out primarily on rats and hamsters (see Gould, 1970. In Trichinosis in man and animals, J. E. Gould (ed.). Charles C Thomas, Springfield, Illinois, pp. 147-189; Ribas-Mujal, 1971. In Pathology of protozoal and helminthic diseases, R. A. Marcial-Rojas (ed.). Williams and Wilkins Co., Baltimore, Maryland, pp. 677-710). A majority of alterations in the chemistry and morphology of muscle (Stewart and Giannini, 1981, Exp. Parasitol. in press) and in blood chemistry (Stewart et al., 1978, Exp. Parasitol. 45: 287-297) in trichinous mice occur during the first 30 days postinfection (PI). In the present investigation we examined by quantitative means the development of myositis during the first 30 days following infection per os of 6to 8-week-old male CD-1 Swiss white mice with 500 infective larvae of Trichinella spiralis. Smith and Castro (1978, Am. J. Physiol. 234: R72-R79) demonstrated that inflammation in the mucosa of the small intestine of the rat accompanying infection with T. spiralis could be quantified by determining peroxidase activity in the mucosa. These same authors demonstrated a positive correlation between peroxidase activity and the appearz