Hydroxyindole-O-methyl transferase activity for the synthesis of 5-methoxyindoles was investigated in the pineal gland of 28 day old male Wistar rats after exposure to white, red and green light for 12 hours daily. It could be demonstrated that, in comparison to white light, red light causes a shift of HIOMT activity for the synthesis of melatonin/5-methoxytryptophol towards an earlier period being highest between 12 and 16 hours. The synthesis of 5-methoxytryptamine and of 5-methoxytryptophan is shifted to 16 hours, while the synthesis of 5-methoxyindole-3-acetic acid, which normally peaks at 16 hours, is increased at this same hour. Green light shifts HIOMT activity for the synthesis of melatonin/5-method being highest between 12 and 16 hours. The synthesis of 5-methoxytryptamine and of 5-methoxytryptophan is shifted to 16 hours, while the synthesis of 5-methoxyindole-3-acetic acid, which normally peaks at 16 hours, is increased at this same hour. Green light shifts HIOMT activity for the synthesis of melatonin/5-method being highest between 12 and 16 hours. The synthesis of 5-methoxytryptamine and of 5-methoxytryptophan is shifted to 16 hours, while the synthesis of 5-methoxyindole-3-acetic acid, which normally peaks at 16 hours, is increased at this same hour. Green light shifts HIOMT activity for the synthesis of melatonin/5-methoxytryptophol to a later period, showing a peak at 24 hours. The synthesis of 5-methoxytryptamine is significantly increased at 24 hours. An increase is also observed in the synthesis of 5-methoxyindole-3-acetic acid at 12 and at 4 hours, times at which this synthesis is also maximal using white light, whereas the synthesis of 5-methoxytryptophan is generally decreased. A possible relationship between the present results and those obtained after exposure to different wavelengths of light on N-acetyltransferase activity, the effect of pterins on HIOMT activity and the effect of different wavelengths on gonadal growth are discussed.