Retinas isolated from rabbits aged less than eight hours to adult were maintained in a flowing physiological medium. The electroretinogram or activity of single ganglion cells were recorded, and receptive fields were studied using stimulation of the retina with focused light. Retinal activity was stable for at least eight hours of incubation. Retinal ganglion cells are electrophysiologically active on the first day of life. They generate spontaneous bursts of action potentials at rates of 10 to 30 spikes/sec, separated by silent intervals of one to six minutes. Maintained trains of action potentials follow elevation of the concentration of K+ in the incubating medium to 10 mM. Ganglion cells are also stimulated by acetylcholine, with apparent thresholds equal to or lower than those of ganglion cells in adult retinas. The first response of the retina to light is a small cornea-negative transretinal potential at day 6, presumably PIII of the electroretinogram. Responses of the ganglion cells are seen at eight days, but the responses are weak and adapt quickly to repeated stimulation. Many unresponsive cells are present. By ten days 60% of ganglion cells respond to light, and examples of mature receptive fields are present. Immature receptive fields at ten days fall into two rough classes, one characterized by a large responsive area with no antagonistic surround, and a second in which the surround can suppress the response to illumination of the center but can not itself cause a discharge. Immature fields are progressively replaced by mature ones, and by 20 days the qualitative organization of receptive fields is indistinguishable from adult.