BACKGROUND : Melatonin regulates metabolism and metabolism related hormones in mammalians. Castration has some adverse effects on the metabolic hormones of dog. This study was conducted to determine the effects of oral melatonin administration on metabolic hormones, as well as to compare changes of these hormones after administration of melatonin in castrated and intact dogs. Twenty healthy mixed breed mature male dogs were divided randomly into four groups (n = 5): melatonin (3 mg/10 kg(, castrated, castrated and melatonin treated, and negative control. Blood sample was collected from jugular vein weekly for 1 month. RESULTS : T3 and T4 hormones had a significant decrease within 1 month following administration of melatonin. No significant change was observed in concentration of FT3 and FT4 hormones. Leptin and ghrelin hormones also had a significant decrease in this period. Leptin and ghrelin had a more significant decrease in "non-castrated and melatonin treated" group compared to "castrated and melatonin treated" group. Galanin had a significant decrease but this neurotransmitter had no significant change in "non-castrated and melatonin treated" group in comparison to "castrated and melatonin treated" group. CONCLUSIONS : It seems that daily administration of melatonin capsule in all dogs can probably decrease concentration of T3 and T4 hormones and balance other metabolic hormones following castration. METHODS : The dogs underwent castration, melatonin treatment and blood sampling.