A "compartment nucleus" consists of a group of spinal motoneurons that supply the individual primary branches of a muscle nerve. Each primary muscle nerve branch innervates a specific subvolume or compartment of a muscle. The spatial and size distributions of motoneurons supplying primary nerve branches of the cat medial gastrocnemius muscle were examined to: determine whether or not the medial gastrocnemius motor nucleus is organized in the same manner as that of the lateral gastrocnemius; determine the type of organization which collectively governs the triceps surae (lateral and medial gastrocnemius, soleus) motor nuclei. Individual primary branches of the medial gastrocnemius nerve were exposed to a horseradish peroxidase solution to retrogradely label their cells of origin. Each compartment nucleus was compared with the distribution of the entire medial gastrocnemius motor nucleus. These data were also compared with data on the spatial and size distribution of motoneurons in the lateral gastrocnemius motor nucleus. Results indicate rostrocaudal overlap in the distribution of motoneurons in each compartment nucleus. However, a rostral to caudal topographic arrangement and preferential size distribution for each compartment nucleus exist for proximal to distal muscle compartments. These and other results indicate that the motor nuclei of the entire triceps surae muscle group are organized by a topologic type map.