Continuous light and dark is caused changes in the Harderian glands. In this study, adult male rats exposed con-tinuous light and dark during the 1,7,15 and 30 days and the structural changes of the Harderian gland examined. Con-tinuous light and dark cause changes in structure of the glans. The structure of the glands of the groups exposed con-tinuous light had most effected. In the light exposed, in the l day, tubule lumina were enlarged and contained numerous porphyrin accreations, the high of the tubules of the glands were decreased while in the 7,15 and 30 days were in-creased connective tissue cell. Therefore, loss of the porphyrin were showed. In the 30 days, the structure of the glands was completely breaked down and degenerated. In the dark exposed, structure of the glands did not usually observed alterations. Only gland tubulles were more hypertrofied. In this study, we provide experimental evidence that changes in the rhythm of the continuous light and dark photoperiod have considerable effects on Harderian glands.