Motoneurons supplying identified muscle groups in the mouse spinal cord were labelled by retrograde transport of horseradish peroxidase. The size of motoneurons was estimated by measuring perimeter and cross-sectional area at the level of the nucleolus for the following seven major muscle groups: quadriceps femoris, adductors and gracilis, gluteal musculature, hamstring muscles, posterior crural musculature, anterolateral crural musculature and intrinsic musculature of the foot. The qualitative observation of two size ranges of motoneuron was supported by the measurements. Frequency distribution histograms of motoneuronal cross sectional area were bimodal for all motoneuronal groups except for the foot musculature. The population parameters and proportions for the six bimodal histograms were estimated by the method of maximum likelihood. It was found that the mean area of the small neuron component, which were presumed to be gamma motoneurons, was similar for the six bimodal systems. In contrast to this the mean area of the large neuron component, presumed to be alpha motoneurons, was found to be different for the six bimodal systems; motoneurons supplying more proximal muscles showed a larger mean area than those supplying distal muscles. The mean area of both components was unaffected by survival time and this was interpreted as indicating that changes in survival time did not label greater numbers of small or large motoneurons. The proportion of motoneurons in the small neuron component was found to vary from 9 to 27%.