: OBJECTIVES: Results of the majority of studies have revealed that diurnal changes in circulating melatonin level in the domestic pig differ from the typical patterns observed in other species. The aim of the present investigation was to study the effect of continuous darkness and continuous illumination on plasma melatonin in the pig. MATERIAL AND METHODS: The study was performed on three groups of immature gilts. The first group was kept under 14hrs light:10hrs dark cycle (500 lx of fluorescent light during photophase), the second group-under continuous illumination (500 lx of fluorescent light) and the third group-under red light with intensity below 1 lx, which was considered as darkness. The pigs were maintained nine days under above reported conditions and then plasma melatonin was monitored during five consecutive days. RESULTS: The diurnal changes in plasma melatonin concentration with increased levels during scotophases were observed in gilts kept under light:dark cycle, but these changes were characterized by low regularity and repeatability. In pigs kept under continuous darkness the circadian changes in plasma melatonin with the highest levels during natural nights were found. No significant circadian changes in plasma melatonin were noted in gilts exposed to continuous illumination. CONCLUSIONS: The obtained results suggest that the diurnal rhythm of melatonin secretion in the domestic pig is generated endogenously and entrained by environmental light. The present results supported also the previously reported observations showing low regularity of diurnal changes in circulating melatonin in the pig.