A well marked diurnal rhythm of hypothalamic monoamine oxidase (MAO) activity was observed inChanna punctatusin LD 11:13 and ambient temperature (15±1°C) with peak activity at mid‐dark phase. Long photoperiod and higher temperature (16L:8D; 22±1°C) induced a 180° shift in the diurnal pattern of the enzyme activity with significantly raised levels at 7.00 and 12.00 h in the photophase. Melatonin‐induced reduction of MAO activity was significant only in the early and mid‐scotophase under the 11L:13D; 15±1°C regime. However, in the 16L:8D; 22±1°C group, after melatonin administration the enzyme activity was considerably reduced at 7 and 12 h in the photophase as also at late scotophase.