Abstract 1. 1. In European green frogs, blinded and with interrupted pineal tracts, the influences of red and of blue light and of darkness on the secretory activity of the subcommissural organ (SCO) were investigated. 2. 2. Three parameters of secretory activity were measured: the amounts within the SCO of stained secretory material and of secretory material labelled by a radioactive precursor and the growth rate of the liquor fibre. 3. 3. Clear differences exist as to the amount of radioactively labelled secretory material. This amount is lower in animals exposed to red light than in animals exposed to blue light as well as in control animals exposed to darkness. 4. 4. The differences between the two light-treated groups of animals can be explained by a different penetration power of the photons from the light sources used and/or by the spectral sensitivity of the unknown photoreceptor involved. 5. 5. The differences between the light-treated animals, on the one hand, and the darkness-treated animals, on the other hand, support the idea of the existence of a deep diencephalic photosensitive structure.