To examine whether lighting conditions during the development of the rat circadian system affect the morphology of the suprachiasmatic nucleus (SCN), three groups of rats were born and maintained until they were 24 days old under constant light (LL), constant darkness (DD) or 24-h light-dark cycles (LD, 12-h light and 12-h darkness). We applied a stereological method to study whether these conditions lead to alterations in the volume of the SCN and changes in the total number of neurons and glial cells. While lighting conditions did not induce differences in the SCN volume, the number of both neurons and glial cells did differ between groups. The DD rats showed the lowest number of neurons. Glial cells were also lower in this group than in the other two groups; however the number of glial cells in LL rats was lower than in LD rats. Moreover, females had more glial cells than males but males and females showed a similar number of neurons. These findings indicate the plasticity of the SCN in response to lighting conditions during the developmental stage.