The principal hormone of the pineal gland, melatonin, is involved in the control of circadian rhythms in birds. Moreover, several studies link melatonin with modulation of metabolism and growth in chickens. Therefore, the present study examines effects of pharmacological doses of melatonin on feed conversion and heat production in female broiler chickens. The supplementation of melatonin at a dose of 150mg/kg of food increased plasma hormone levels to 28.76 +/- 0.47 ng/ml and 19.29 +/- 0.62 ng/ml in 14- and 21-day-old chickens, respectively. The treatment resulted in a highly significant decline in heat production of 2-week and 3-week-old chickens (8.38 and 13.05%, respectively). Feed conversion was improved only in younger chickens. The present data are consistent with hypothesis that melatonin reduces heat production by lowering body temperature and regulating heat dissipation.