In 1694 Johann Jacob Harder described, for the first time, the harderian gland, located near the eye in the great majority of vertebrates, and regarded it as a lacrimal gland. This gland has multiple functions that vary according to the animal species. In the present work, we review the literature on the harderian gland of rats, focusing our study on the detection of great amounts of protoporphyrin IX (PpIX) produced by rodent glands. Protoporphyrin IX is a powerful photosensitizer that is widely used for photodynamic therapy (PDT). We also discuss the anatomic and the histological evidence for the presence of PpIX in the harderian gland of Wistar rats. Protoporphyrin IX has been detected in the lumen and in acinar cells of this gland, as confirmed by fluorescence microscopy. These findings together with numerous reports in the literature suggest that the harderian gland could be useful experimental model for studying the photodynamic process.