The circadian expression of the serotonin N-acetyltransferase (NAT) gene in the rat retina is considered to be generated by an endogenous retinal clock. To investigate the developmental aspects of the clock system in the mammalian retina, we determined daily expression patterns of NAT mRNA in the retina of postnatal rats. Animals were kept under a daily light-dark cycle (LD), or transferred to constant darkness (DD) from LD, and retinal mRNA levels were examined by Northern blot analysis. NAT mRNA had begun to show a daily change in LD as early as postnatal day 2 (P2). Whereas rhythmic variations in DD were not observed until P14, at P20, a circadian oscillation had already occurred. These findings suggest that in the rat retina, even though a daily LD induced rhythmic changes in NAT mRNA levels at the early postnatal stage, it was after P14 that the retinal clock system began to regulate the circadian NAT expression.