Pineal hydroxyindole-O-methyl transferase activity was measured in growing rats as an estimate of melatonin production during maturation. No significant HIOMT was found at 10 days or earlier. At 14 days HIOMT/gland was 40% of the 8- to 10-week value. Sympathetic innervation, changes in vascularization and metabolite concentrations were discussed as possible mechanisms initiating production of the enzyme. If total HIOMT activity was compared to body weight, a dramatic decrease in relative activity was observed to develop after a peak at 21 days. This was interpreted to suggest a relative decrease in melatonin production occurs after 21 days which might permit previously inhibited sexual maturation to develop. (Endocrinology84: 1523, 1969)
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Klein et al. (1969) studied this question.