The cell bodies and dendrites of cat spinal alpha-motoneurons were studied after intracellular staining with horseradish peroxidase. The mean diameter of the soma was positively correlated to both the mean diameter and the combined diameter of the first-order dendrites, but not to the number of first-order dendrites. On the average, 11.2 dendrites originated from the soma. The dendritic trees were more extensive than has been described previously. The mean value for the combined length of a whole dendrite was 4.7 mm, while the mean values for the total surface area and volume of a dendrite were 33.0 x 10(3) micron (2) and 27.2 x 10(3) micron (3), respectively. The diameter of the first-order dendrite was positively correlated to the combined length of the entire dendrite, the number of dendritic branching points, and the number of dendritic end branches. The diameter of the first-order dendrite was also directly proportional to the volume and the surface area of the entire dendrite. About 75% of the dendritic surface area and 55% of the dendritic volume was located more than 300 micron away from the soma. The dendrites constituted about 97% of the surface area and about 75% of the volume of the entire motoneuron (excluding the axon). The dendritic tapering was moderate. On the average, the distal decrease in dendritic diameters caused a reduction in the combined dendritic parameter (sigma d 3/2) by 1.5% and 15% at 500 micron and 800 micron distance, respectively, from the soma.