After partial ablation of the superior colliculus (tectum) in neonatal hamsters, the whole extent of the visual field comes to be represented in a compressed map on the remaining tectal fragment. However, the total volume of tectal tissue in which retinotectal fibers arborize is less than normal. These observations suggests that the retinal ganglion cells which arborize in this reduced volume might arise throughout the whole extent of the retina but be fewer in number than normal. Alternatively, the ganglion cells which project to the tectum might be normal in number but reduced in terminal arbor size. To distinguish between these possibilities, we have used tectal injections of horseradish peroxidase to label retinal ganglion cells which project to the tectum. The numbers of labelled cells per mm2 of retina were counted in selected regions. In hamsters with small lesions, which left 80--85% of the tectum intact, the density of labelled retinal ganglion cells was normal. However, in hamsters with larger lesions, the density of labelled cells was significantly lower than normal.