A distinct diurnal rhythm in hydroxyindole-O-methyltransferase (HIOMT) activity in the male albino rat.

Pineal hydroxyindole-O-methyltransferase (HIOMT) has been reported to show a diurnal rhythm activity in the rat, although this is not fully confirmed. The present study reexamines this issue and reports a distinct diurnal rhythm in rat pineal HIOMT. Peak enzyme activity occurs 5 hr after the onset of darkness, followed by two lesser bursts of activity preceding each change in the photoperiod. The approximately 18-fold increase in methylating capacity at night represents the greatest variation in HIOMT activity thus far reported in the rat and may be attributed to the age, weight, and housing regimen of the animals used.