The phase of a circadian clock in the chicken pineal gland is reset by an environmental light signal, which is captured by the pineal photoreceptive molecule(s). Here we show that the mRNA level of pinopsin, a predominant photoreceptive molecule in the pineal gland, undergoes a diurnal fluctuation in chickens maintained on a light/dark cycle. The mRNA levels in the light were approximately six times higher than those in the dark. This fluctuation was not observed in constant darkness, where the mRNA levels remained low. Subsequent light exposure of chickens increased the amount of pinopsin mRNA regardless of the circadian time. Clearly, the expression of pinopsin gene is controlled by a light signal, independent of the circadian clock. In vitro experiments using cultured pineal glands isolated from the visual system also revealed the light-dependent increase in pinopsin mRNA level, indicating that the pineal photoreceptive molecule(s) is responsible for the induction. These results demonstrate the presence of a feedback loop through which the light signal captured by pinopsin stimulates the transcription of its own gene in the chicken pineal gland. In contrast, pinopsin protein remained at an almost constant level in chickens maintained under the same light/dark cycles. The protein level, however, decreased to approximately 50% of the light/dark level under constant darkness and subsequently increased upon exposure to light after the dark period. It is suggested that, under the light/dark cycles, the pinopsin protein level is kept constant by the light-dependent synthesis, which would compensate for a possible degradation of pinopsin in the daytime.