The development of the rabbit pineal gland has been studied by light and electron microscopy from the 1st to the 120th postnatal day. After 24 h of postnatal life, the pineal parenchyma is highly cellular, showing two identifiable cell types: pinealocytes I and II. Immature type II pinealocytes arrange either in cellular cords or clusters or forme rosette-like structures. At the 5th postnatal day, corticomedullar differentiation is established. Rosette-like structures and cellular cords are absent from the cortex. Along the postnatal period, nuclei of pinealocytes are set apart due to cytoplasmic widening and development of cell processes. These structures pervade the cellular cords and rosette-like structures formed by immature type II pinealocytes. Rosette-like structures are no longer seen beyond the 30th postnatal day, and cords of type II pinealocytes from the 90th postnatal day on. At this time, the rabbit pineal gland is considered to be histologically mature.