A quantitative light- and electron-microscopic study has been made of the retinae of rats which were exposed to different lighting conditions for between one and 15 weeks in young adulthood, having been reared in identical conditions during development. The width of the inner and outer segments of the photoreceptors and the width of the outer plexiform layer varied inversely with the light intensity under diurnal lighting conditions of 10 h light/14 h dark. Linear regression analysis showed that the widths were inversely related to the fourth root of the light intensity as measured in lux. Both central and peripheral areas of retina showed a similar change. No change was seen in the widths of the inner plexiform layer, or of the inner and outer nuclear cell layers. Nor was there a difference in the packing density or size of the nuclei in the nuclear cell layers. The number of ribbon synapses in the outer plexiform layer also varied inversely with the intensity of diurnal light. Linear regression analysis showed that the number of synapses was inversely correlated with the fourth root of the light intensity and was positively correlated with the width of the outer plexiform layer. The number of ribbon synapses was increased by up to two and a half times in constant darkness compared to diurnal light of 35 lux. The increase was present but not maximal after one week of exposure. The length of synaptic ribbons was unchanged. The nerve terminals forming such synapses were increased in size but not in number. After one week, there was little or no additional change in the retinal widths and number of synaptic ribbons with time. However, there was a progressive increase with time in nerve terminal size (two-fold in area) in constant darkness. There was some evidence of a slight decrease in nerve terminal number and increase in size of retinal nuclei with age. It is concluded that the adult retina responds to a different lighting environment by a relatively rapid change in the size of photoreceptor segments, by a progressive and large change in number of ribbon synapses and by a slower progressive and large change in the size of photoreceptor nerve terminals. The response is quantitatively determined by the strength of the stimulus but not in a linear fashion. These results are compared with the effects of environmental stimulation of other areas of the nervous system.