Field and laboratory experiments were performed to evaluate toxicological responses of Cyprinus carpio exposed to the commercial herbicide clomazone (500 mg l(-1)). Fish were exposed to 0.5 mg l(-1) of the formulated herbicide for 7, 30, and 90 days. Fish were exposed to clomazone in field conditions (7, 30, or 90 days trapped in submersed cages together with rice crops) and in laboratory conditions where the fish were placed in 45-l tanks with tap water only for 7 days. Fish exposed for 7, 30, or 90 days showed no alterations in acetylcholinesterase (AChE) activity under field conditions. Under laboratory conditions, decreased muscle AChE activity was observed only after 7 days of exposure. During the same evaluation period (7 days), oxidative stress parameters changed under both field and laboratory conditions; however, metabolic parameters were altered only under field conditions. Disorders in oxidative stress parameters and metabolism were evident in different tissues up to day 90 after treatment. These overall results show that AChE activity changed only under laboratory conditions. Oxidative stress, along with metabolic parameters, may be good indicators of herbicide contamination in C. carpio under rice-field conditions.