The objective of this study was to investigate the effects of ascorbic acid on the toxicity of cypermethrin's on histopathological lesions in tissues and protein, glycogen levels in Oreochromis niloticus. Nile tilapia was exposed to 0.22 and 0.44 mu g/l cypermethrin + control diet. 0.22 and 0.44 mu g/l cypermethrin + ascorbic acid supplemented diet for 20 days. The fish were allowed recuperation period of 15 days in pesticide-free water and fed with ascorbic acid suplementation diet. In light microscopic investigation, histopathological lesions were observed in the gill, liver and kidney. The severity of lesions accreted depending on increased pesticide concentration and control diet. Some of the lesions were reversible or at least were less pronounced after recuperation period. Protein levels decreased in some groups after treatment period according to control groups (p < 0.05). The highest depletions in liver, muscle and gill protein levels were found in 0.44 mu g/l cypermethrin + ascorbic acid supplemented diet group (62.23%), in 0.22 mu g/l cypermethrin + control diet group (53.12%) and in 0.44 mu g/l cypermethrin + control diet group (61.87%) after 10 days, respectively. These levels increased at the end of the recuperation period. The highest depletion in liver glycogen levels was found in 0.22 mu g/l cypermethrin + control diet group (50.50%) after 10 days (p < 0.05). At the end of recuperation period, there was no difference between the groups (except 0.22 mu g/l cypermethrin + ascorbic acid supplemented diet group) and controls. The decrease of muscle glycogen, except 0.22 mu g/l cypermethrin + ascorbic acid supplemented diet group, was recorded at the end of 10 and 20 days. In the recuperation period, an increase was observed at all groups. These results revealed that the histopathology, protein and glycogen can work as good indicators of stress of a toxicant on fish. Ascorbic acid serves fish as an antitoxic agent against pesticide toxicity. (C) 2009 Elsevier B.V. All rights reserved.