The effect of zinc-carnosine complex (Z-103) on muscle function in dystrophin-deficient (mdx) mice was examined using several different courses of repetitive administration. Z-103 at a dose of 100 mg/kg increased the load resistant time (LRT), during which an animal bearing a load holds himself upright on a wire net, in mdx mice when administered at an age of less than about 4 months. The effect of Z-103 on LRT was independent of sex when given by intraperitoneal (I.P.) administration between 4 and 8 weeks of age. Administration of Z-103 from the age of 4 to 9 weeks had no significant effect on wet weight, magnitude or rate of rise of twitch force, or rate of decay of twitch force over time with twitch elicited by 0.5 Hz of electricity in the extensor digitorum longus muscle or calcium content in the gastrocnemius muscle, while it increased the magnitude of twitch force in the soleus muscle. These results suggest that Z-103 reduces fatigability of the whole body in mdx mice, possibly by increasing the contractility of slow fibers.