Dietary administration of 3.6 mg/kg (BW)/day of pyrimethamine (PYR) to the 10 pregnant Goettingen minipigs for the 1st quarter (during days 11 to 16) or the 2nd quarter (during days 17 to 22) of the organogenetic period elicited the congenital splayleg (CSL) in 20 out of 27 live newborns. Only a splayleg piglet out of 18 was found in the control group. The incidence of stillbirth was higher in the PYR groups than the control group, but no major external malformation was observed in the piglets of the PYR groups. Histological and biochemical examinations of the muscles from the hindleg, foreleg and truncus were carried out. The severity of myofibrillar hypoplasia depended on neither CSL symptoms nor PYR administration. Although a significant decrease of protein content of the muscles was observed in the piglets of the PYR groups, the mechanism underlying the pathogenesis of induced splayleg was obscure.