We analyzed muscular activity of the lower extremities during level walking using positron emission tomography (PET) with (18)F-fluorodeoxyglucose ((18)F-FDG). We examined 17 healthy male subjects; 11 were assigned to a walking group and 6 to a resting group. After (18)F-FDG injection, the walking group subjects walked at a free speed for 15 min. A whole-body image was then obtained by a PET camera, and the standardized uptake ratio (SUR) was computed for each muscle. The SUR for each muscle of the walking group was compared with that for the corresponding muscles in the resting group. The level of muscular activity of all the muscles we examined were higher during level walking than when resting. The activity of the lower leg muscles was higher than that of the thigh muscles during level walking. The muscular activity of the soleus was highest among all the muscles examined. Among the gluteal muscles, the muscular activity of the gluteus minimus was higher than that of the gluteus maximus and gluteus medius. The concurrent validity of measuring muscular activity of the lower extremity during level walking by the PET method using (18)F-FDG was demonstrated.