Melanopsin, an opsin protein expressed in mammalian retinal ganglion cells (RGCs), makes them responsive to light. Such photosensitive RGCs form the retinohypothalamic tract (RHT) that provides signals to the suprachiasmatic nucleus (SCN), the master regulator of circadian rhythms. The SCN is adjusted daily to the environmental day/night cycle by signal inputs incoming from the RHT. In the present work we have studied, using immunohistochemistry techniques, the types and number of cells which expressed melanopsin during the postnatal development of pigmented C3H/He mice maintained in a standard daily cycle (12-h light/12-h dark). Our results clearly show for the first time that the retina maintains a rather constant number of melanopsin-expressing RGCs from the first postnatal day and, thus, demonstrate that no loss of these photosensitive cells occurs during postnatal development. This supports the general idea that the non-image-forming system, in which these cells are involved, is functional at the very early postnatal stage.