Abstract: Avian pineals show rhythmic, circadian melatonin secretion pattern also in vitro. The phase of this rhythm can be modified by changing the illumination. Reversed in vitro illumination reverses the phase of the circadian melatonin rhythm in two days. In the present study the effect of the wavelength on the speed of the phase shift of the melatonin rhythm of the explanted chicken pineals was investigated in a dynamic in vitro bioassay. Chicken pineals were placed into perifusion columns and the tissue was exposed to reversed illumination through various light filters. Changes in the melatonin release during 4 day long experiments were studied. Clear differences in the speed of the reversal of the melatonin rhythm were observed as a function of the transmission spectrum of the light filters. The shorter the transmitted wavelength, the more rapid the phase shift was found. These data are in good agreement with earlier studies showing that the chicken pineal photo-pigment, pinopsin, is a blue light sensitive molecule. Our observation reveals that the blue light sensitive pigment is not only present but also fully functional in controlling the circadian biological clock in the chicken pineal gland.