The aim of this study was to examine the involvement of melatonin in the photoperiod-related development of inflammation in chickens. Newly hatched birds obtained in summer and winter were kept in a light : dark (LD) cycle (corresponding to the season of hatch) or in continuous light (LL). Half of the chickens held under LL conditions were given melatonin during the subjective night. Peritonitis was evoked on the 12^th day of life by thioglycollate (TG) injection at the beginning or towards the end of the subjective day and the effect was measured 4 h later, i.e. during the day or at night. The development of inflammation was evaluated by the number of peritoneal leukocytes (PTLs) inflowing to the peritoneal cavity and by the level of mRNAs encoding proinflammatory cytokines in the blood leukocytes. Day/night differences in PTL number were seen only in the summer, regardless of the experimental conditions. In chickens kept under LL conditions and given melatonin, TG injection caused an increase in interleukin-6 (IL-6) mRNA level in blood leukocytes in summer and a decrease in winter. Neither experimental conditions nor circadian time affected the interleukin-18 (IL-18) mRNA level in summer, whereas in winter abundance of this transcript exhibited the daily variations and the influence of the treatment. The obtained results suggest the existence of clockcontrolled diurnal and seasonal variability of the chicken inflammatory reaction, which is influenced in different ways by circulating melatonin.