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These studies were conducted to evaluate the effects of different photoperiods on performance and immunity of broiler chicks and investigate if its immunomodulation effects were due to the melatonin or not. In experiment 1, two hundred eighty eight one-day-old commercial broiler chicks were placed into floor pen between constant lighting(CL) and intermittent lighting(IL, 1h light : 3h darkness) for 6 weeks. Weight gain, feed intake and feed conversion ratio were weekly examined. Lymphocyte activities, antibody response to Newcastle disease virus vaccine(NDV) and sheep red blood cell(SRBC) and IgG concentration were measured at the end of experiment. Changes of melatonin level were measured at 5 weeks in sera. In experiment 2, one hundred ninety two two-week-old male broiler chicks were subcutaneously injected with 0.1㎖ melatonin for 3 weeks. Lymphocyte activities, antibody response to NDV and SRBC, IgG and melatonin concentration were measured at 5 wk of age. In experiment 1, Chicks illuminated IL showed significantly higher body weight and lowest feed conversion ratio than CL treatments at 7 wk of age(P$lt;0.05). Melatonin concentration changed by dark-light cycle. Lymphocyte activity of chickens illuminated IL was significantly higher compared to That of CL treatments(P$lt;0.05). In experiment 2, increasement of antibody response to SRBC and lymphocyte activity were also observed. As the result, IL treatment improved performance and immunity of broiler chicks and affected melatonin concentration in sera. Blood melatonin were also related with melatonin injection. This results indicated that melatonin may have a role in immunomodulation in broiler chicks. The results of these experiments showed that antibody response to SRBC and total IgG concentration was enhanced by different photoperiod and melatonin treatments.
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