In cats anesthetized with chloralose-urethane, microelectrodes were inserted into the optic tract on one side, and single fiber responses were recorded to light and to electrical stimulation delivered to the other side. The responses occurred after cessation of (usually) 100 msecond flashes in 48 of 63 units. Thirty-nine of them could be adequately described. Three types of post-flash response were seen: intensity-sensitive (16), not sensitive to intensity (17) and mixed (6). There was no correlation between the type of response to light and the conduction velocity along the axon. All post-flash portions of on-off responses were either not intensity-sensitive or showed mixed forms of response. The intensity-sensitive units had the usual characteristics of off responses. They showed absence of activity during the flash and a waning discharge starting about 35-90 mseconds after its cessation (blue flashes of maximum and threshold intensity respectively). The number of spikes increased (increased frequency and prolonged duration of discharge) with increasing amounts of light above threshold. When the duration of a flash of constant intensity was changed, there was a nearly linear relationship between spike number and log flash duration up to 1-40 mseconds. The latency tended to remain constant except at extremes of intensity or duration; when it changed it was in the direction of an increase with decreasing intensity. At flash durations of about 1-40 mseconds,it was the latency to the on of the flash which was constant. Thresholds tended to be low and there was no spontaneous activity. No correlations with color apart from light intensity could be found. Units not sensitive to intensity were silent during and up to 500 mseconds after a (usually) 100 msecond flash. The spike discharge was sparse, irregular and variable from flash to flash. There was no close relationship between spike number and light intensity. The threshold was usually high and there was no spontaneous activity. The latency of some increased and of some decreased with decreasing intensity. Mixed responses were characterized by 2 bursts, the 1st intensity-sensitive and the 2d not. As the latency of the 1st increased with decreasing intensity, that of the 2d decreased in large steps. The threshold of the 2d burst was usually low, and there was spontaneous activity. One or the other component of the mixed burst could be absent when the flash was red or blue, present in response to the other color.