The sciatic nerve of the rat is a commonly used model for studies on nerve injury, regeneration, and recovery of function. To interpret the changes that occur in a neuron population subsequent to peripheral nerve injury, and to compare different repair procedures, it is essential to have a complete and accurate understanding of the population's normal cellular constituents and their locations. This study reports on the numbers, sizes, and topographic distributions of the motoneuron populations of individual branches of the rat sciatic nerve (peroneal, tibial, sural, and the medial and lateral gastrocnemius nerves), as determined by retrograde transport of HRP (or WGA-HRP) from cut proximal nerve ends isolated in wax to prevent spread of the tracer substance. Optimal labeling of motoneurons was evident between 42 and 73 h of survival. Reconstructions were made from 40-micron serial sections of spinal segments L6 through L2, usually in the coronal plane. Accurate motoneuron counts were obtained by detailed reconstructions in which an accounting of all "split cell" fragments was made to avoid double cell counts. The sciatic nerve of the albino rat contains a total population of about 2005 +/- 89 motoneurons. The tibial nerve contained 982 +/- 36 cells or 49% of the total. The common peroneal nerve contained 31% or 632 +/- 27 motoneurons. The medial and lateral gastrocnemius nerve branches contained collectively 322 +/- 16 (16%). The sural nerve accounted for only 68 +/- 10 motoneurons (3%). The sciatic motoneurons form a continuous, compact cell column in the dorsolateral quadrant of the ventral horn extending from rostral L6 into the caudal third of L3 over a longitudinal distance of about 6.3 to 7.5 mm. This fusiform column shows its greatest width, 0.5 mm, in mid-L4. Within this compartment motoneurons of each branch of the sciatic occupy spatially distinct subcompartments. Their relative positions are described in detail.