Using a method in which no substrate is added to the incubation medium, the capacity of HIOMT to synthesize 5-methoxytryptophan, 5-methoxytryptamine, 5-methoxyindole-3-acetic acid, 5-methoxytryptophol and melatonin has been determined in the pineal and the eyes of the mole, a mammal having an atrophied visual system. The results demonstrate that the indoleamine metabolism in the retina is similar to the indoleamine metabolism in the pineal. Moreover, in all animals studied both eyes together synthesize 2 to 30 times more of 5-methoxyindoles than the pineal, a result which proves again that the pineal is not the only and not always the most important source of 5-methoxyindoles. With the exception of 5-methoxyindole-3-acetic acid, the synthesis of which is practically neglectable, the production of all other 5-methoxyindoles in the pineal as well as in the retinae is always larger than that of melatonin. In the pineal, 5-methoxytryptophan, for example, is synthesized in a quantity which is 60 to 170 times larger than that of melatonin, while in the retinae the synthesized amount of 5-methoxytryptophan is even 60 to 1000 times larger than that of melatonin.

N-acetyltransferase activity was quantified in the pineal gland of 38 day-old male Wistar rats under different experimental conditions. Under daily white light/dark photoperiods (12 L : 12 D), N-acetyltransferase activity showed a peak of activity at midnight for the synthesis of both N-acetylserotonin and melatonin. Using red light instead of white light a shift of the enzyme activity involved in the production of both compounds towards the preceding light period was observed. Using green light instead of white light the enzyme activity shifted to later periods. Under white light/dark conditions pineal O-acetyltransferase activity when quantified was maximal during the early hours of the night. With periodic red light this maximal activity was extended over a longer period including late hours of the preceding day and later hours of the night. Periodic illumination with green light caused a peak of activity at midnight and an increase of activity at then end of the dark period. A possible relationship between previous results obtained on the influence of pterins on HIOMT activity and the effects of light on reproduction is discussed.