The discovery of a subtype of retinal ganglion cells (RGCs) utilizing a novel opsin, melanopsin, as a photopigment to generate intrinsic light responses, has allowed rapid progress in the past decade towards understanding the non-image forming visual system. The mcRGCs (for melanopsin-containing) or ipRGCs (for intrinsically-photosensitive) are the earliest light responsive neurons in the retina. Similar to many other types of RGCs, mcRGCs are overproduced and some eliminated in early development. However, the mechanisms controlling the number of mcRGCs in early postnatal development are not clear. We report that in the rat retina, constant light exposure down regulated melanopsin expression, but significantly and permanently increased the number of mcRGCs but not neurofilament positive RGCs in adult in first postnatal week. BrdU and retrograde labeling experiments ruled out possibilities that constant light exposure induced proliferation of mcRGCs or conversion of other types of RGCs into mcRGCs. Daily intravitreous injection of TTX completely abolished the light induced increase of mcRGCs. These findings strongly indicate that the apoptosis of mcRGCs is suppressed by the intrinsic photosensitive spiking activity.