We examined atrogin-1/MAFbx mRNA expression in the smooth muscle of gizzards from egg- and meat-type chickens. Gizzard weight relative to body weight was significantly lower in the meat-type chickens than in the egg-type at 14 d of age. In contrast, the level of atrogin-1/MAFbx mRNA in the gizzard was significantly higher in the meat-type chickens than in the egg-type chickens. Thus atrogin-1/MAFbx mRNA expression in the smooth muscle of the gizzard was higher in meat-type chickens than in egg-type chickens, in contrast to its expression in the skeletal muscles.