The influence of three pterin derivatives on the diurnal fluctuations of HIOMT (hydroxyindole-O-methyl transferase) activity was studied in the isolated pineal glands of 42-day old male Wistar rats during the month of October. The method used permitted the separate determination of four HIOMT activities. --Reduced neopterin stimulated the methylation of the substances, 5-HTP, 5-HT and 5-HIAA (see abbreviations in Material and methods), during the night. HIOMT action on the combinations N-Ac-5-HT/5-HTL was shifted to a later moment in the dark period. --Pterin-6-aldehyde stimulated HIOMT action on 5-HT during the daytime. HIOMT action on the substrates 5-HTP, 5-HIAA and N-Ac-5-HT/5-HTL was shifted towards an earlier period. --Isoxanthopterin did not exert any influence on diurnal variation in the four HIOMT activities. It may be concluded that reduced neopterin and pterin-6-aldehyde influenced the activity and the circadian rhythmicity of 5-methoxyindole synthesis. Those alterations might be important in the regulation of reproduction.