The efficacy of melatonin co-administration on aflatoxicosis in chicks was investigated. Ross PM3 breed chicks were divided into groups of 10 and given conventional feed. One of the groups was kept as a control (C), and the others were given 150 ppb aflatoxin (AF1), 300 ppb aflatoxin (AF2), 150 ppb aflatoxin plus 10 mg/kg/bwt melatonin (AF1+M), 300 ppb aflatoxin plus 10 mg/kg/bwt melatonin (AF2+M), 10 mg/kg/bwt melatonin (M), and 1% ethanol (E). After 21 day-treatment period, the chicks were sacrificed, liver and kidney tissues were collected, processed for immuno-histochemical staining, in situ TUNEL method, and biochemical analyses. Vacuolar degeneration, necrosis, bile duct hyperplasia in liver, and mild tubular degeneration in kidney were detected in AF groups. Pathological changes were markedly reduced in AF+M groups, and a microscopic view similar to group C was observed. Increased immunoreactivity against inducible nitric oxide synthase (iNOS) and nitrotyrosine was detected in AF groups compared to weak immunoreactivity in group C. Immunoreactivity in AF+M groups was markedly reduced compared to AF groups and was similar to group C in liver and kidney. Many apoptotic cells were detected in the livers of AF groups, whereas there were no apoptotic cells in AF+M groups. While reduced glutathione (GSH) levels in liver and kidney of AF groups were greatly reduced, malondialdehyde (MDA) levels increased. With melatonin co-administration, the levels of GSH and MDA approached to the values of group C. These results indicated that nitrosative tissue degeneration caused by aflatoxin could be greatly reduced by melatonin supplementation in chicks. © 2008 Elsevier Ltd. All rights reserved.