Lampreys have a complex life cycle, with largely differentiated larval and adult periods. Despite the considerable interest of lampreys for understanding vertebrate evolution, knowledge of the early development of their eye and pineal complex is very scarce. Here, the early immunocytochemical organization of the pineal complex and retina of the sea lamprey was studied by use of antibodies against proliferating cell nuclear antigen (PCNA), opsin, serotonin, and γ-aminobutyric acid (GABA). Cell differentiation in the retina, pineal organ, and habenula begins in prolarvae, as shown by the appearance of PCNA-negative cells, whereas differentiation of the parapineal vesicle was delayed until the larval period. In medium-sized to large larvae, PCNA-immunoreactive (-ir) cells were numerous in regions of the lateral retina near the differentiated part of the larval retina (central retina). A late-proliferating region was observed in the right habenula. Opsin immunoreactivity appears in the pineal vesicle of early prolarvae and 3 or 4 days later in the retina. In the parapineal organ, opsin immunoreactivity was observed only in large larvae. In the pineal organ, serotonin immunoreactivity was first observed in late prolarvae in photoreceptive (photoneuroendocrine) cells, whereas only a few of these cells appeared in the parapineal organ of large larvae. No serotonin immunoreactivity was observed in the larval retina. GABA immunoreactivity appeared earlier in the retina than in the pineal complex. No GABA-ir perikaryon was observed in the retina of larval lampreys, although a few GABA-ir centrifugal fibers innervate the inner retina in late prolarvae. First GABA-ir ganglion cells occur in the pineal organ of 15-17 mm larvae, and their number increases during the larval period. The only GABA-ir structures observed in the parapineal ganglion of larvae were afferent fibers, which appeared rather late in development. The time sequence of development in these photoreceptive structures is rather different from that observed in teleosts and other vertebrates. This suggests that the unusual development of the three photoreceptive organs in lampreys reflects specialization for their different functions during the larval and adult periods. © 2001 Wiley-Liss, Inc.