The morphology of the ganglion cells of the monkey's retina was revealed by filling the cells with horseradish peroxidase from their cut axons in the optic nerve. This procedure gave much more consistent results than the Golgi method, was much quicker and filled dendrites just as extensively. Quantitative measures of the dendritic tree of two types of ganglion cell, P alpha and P beta, suggest that they correspond to the physiologically defined Y- and X-cells, respectively.