Hydroxyindole-O-methyltransferase (HIOMT) activity was observed in the pineal organ and the retina of the lateral eyes of Lampropholas guichenoti. No activity was recorded from the parietal eye of this species. There was a distinct diurnal pattern to the changes in HIOMT activity in both the pineal organ and the retina, there being a peak of activity 2 hr after the onset of darkness and another greater burst of activity commencing 1 hr before the onset of light. When lizards were exposed to an artificial lighting regime which was different from their normal environmental lighting the peak of HIOMT activity associated with the onset of darkness was established after 5 days, whereas that associated with the start of the light period took approximately 12 days to appear. It is concluded from these results that the pineal of Lampropholas is capable of setting itself to a particular photoperiod and is capable of measuring a lapse of time in order to do so.