The development of pineal organ in fetal and newborn Boscat rabbit was studied by examination of serial histologic sections. The sections were stained with Gallocyanin, H & E, iron HX, van Gieson, orcein, silver impregnation for reticular fibers and PAS. It was found that, the first pineal anlage appeared in 14 days-old embryo as a midline evagination of the roof of the third ventricle. The pineal analge increased rapidly in size. The neuro-epithelium bordering the evagination underwent mitosis and gaves rise to pinealocytes and glial cells. Differentiation into these two types started to present at 22 days and can be distinguished in 24 days embryo. The growth increased rapidly in size and acquired a compact appearance. Its connection with the third ventricle was shallow and short and known as pineal recess. So, a large surface area of the pineal at this site was exposed to the ventricular CSF. From 24 days of gestation, differenitiation of the pineal into three parts was evident. The organ possessed 3 parts; (1) a superfacial or distal part, located in the vicinity of confluens sinuses, with its floor sharing in the formation of pinal dorsal capsule (2) a deep or proximal part of considerable size with an intimate relation to the third ventricle via the shallow pineal recess and the long suprapineal recess lined with highly convoluted choroid plexus (3) a thick voluminous stlak connecting the deep and superfacial part. Histologically, the pineal was formed of stroma derived from menin geal mesanchyma. The only connective tissue fibers that could be detec ted were the reticular fibers. These were PAS positive. The parenchyma was formed mainly of pinealocytes with pleomorphic vesicular nuclei, and in Bouscat embryos showed characterestic rosette-shaped arrang ment. Glial cells were few, and evenly distributed throughout the organ. The organ was characterestically surrounded by abundant vascular channels, the great cereberal, vein, the transverse sinuses, and the confluens sinuses. The functional significance of the relationship of the organ and nearby structures are discussed.