In order to clarify the morphological features of peripheral motor nerves and motoneurons that innervate trunk muscles, the size distribution of external oblique (EO) peripheral motor fibers and motoneurons of the thoracic and the lumbar segments were examined. Histograms of the size distribution of EO motor fibers in peripheral nerves after ganglionectomy clearly had a bimodal distribution of small fiber groups and large fiber groups. It is very likely that small fiber groups correspond to gamma motor fibers and large fiber groups to alpha motor fibers. Gamma and alpha motor fiber groups were separated at 8-14 microm. The average diameter of the gamma and alpha motor fibers were different in each segment. The ratio of gamma and alpha motor fibers was approximately 1:2.0 in the thoracic segments and from 1:1.8 to 1:0.9 in the lumbar segments. Horseradish peroxidase was applied to the central stump of EO nerves, and the size distribution of EO motoneuron cell bodies in the thoracic and the lumbar spinal cords was examined. The size distribution of motoneuron cell bodies was bimodal in one cat (small and large motoneurons) and unimodal in three cats. When the ratio of small motor fibers to large motor fibers in peripheral nerves was applied to that of small motoneurons to large motoneurons, the separation of small and large motoneurons was approximately 40 microm. These results suggest that the morphological characteristics in peripheral nerves of trunk muscles are not reflected in motoneurons.