Evidence has been accumulating which supports the hypothesis that through phylogenetic evolution, the mammalian pinealocyte has evolved from the pineal photoreceptor-cell which is characteristic of certain lower vertebrates. In the present study, developing pineal glands in fetal rats and hamsters were studied at light and electron microscopic levels to determine whether or not the ontogenetic development of pineal cells affords clues to their phylogenetic origin. Pineal development in fetal rats occurs during the last eight days of gestation (14–22) and in fetal hamsters, during the last five days of gestation (11–16). The pineal gland exhibits many similarities in the two species as it develops from a tubular evagination of the diencephalic roof into a compact cellular mass. Different cellular types could not be readily distinguished in fetal material from either species. In both animals, as long as lumina persist, cells bordering on these lumina exhibit surface modifications reminiscent of developing photoreceptor-cells, i.e., an “ellipsoid-like” apical cytoplasmic bulge which often contains one or two centrioles and bears a ciliary derivative exhibiting a 9 + 0 tubular configuration. As pineal tissue proliferates, the lumina and cilium-like structures disappear. The findings, when considered with phylogenetic studies, strengthen the hypothesis that the mammalian pineal gland contains cells derived from the photoreceptor-cell line.