Effects of pregnancy stimulation upon histochemically assessed myofibrillar ATPase and muscle fiber diameters were analysed in the rectus abdominis (RA) muscle of guinea pig. Samples of the muscle were taken at 30, 40, 50, 60, and 70 days of pregnancy and compared with samples of the same muscle taken from nonpregnant guinea pigs. Changes in muscle fiber proportions were noted through the course of pregnancy. Starting from 50 days of gestation an increase in type I fibers and a decrease in type IIB fibers were noted. Increase in muscle fiber diameters was also observed in type I, IIA, and IIB fibers. In addition, the RA muscle of the male guinea pig was compared with that of the female guinea pig and showed more type IIA and less type IIB fibers and all the three fiber types were larger than those of the female.