Nile tilapias (Oreochromis niloticus) of three groups with different weights (juvenile, adults of lower and higher weight) were exposed to benzo[a]pyrene (0.5 mg.L-1), cupper (0.5 mg.L-1), cadmium (0.5 mg.L-1) and diazinon (1.0 mg.L-1), for 72 h. In order to determine how animals of the same species respond to such contaminants over weight gain, biochemical biomarkers (glutathione S-transferase, superoxide dismutase, catalase, glutathione peroxidase, acetylcholinesterase, carboxylesterase and lipid peroxidation levels) were analyzed in liver and in gills. In the juvenile group the catalase was induced by exposure to cupper in gills, while lipid peroxidation levels were low. In the same group the glutathione S-transferase was induced in gills, likewise glutathione peroxidase was induced by diazinon in liver. In gills of the group of adults of lower weight the low lipid peroxidation levels, in exposure to diazinon treatment, may be related to inhibition of carboxylesterase. In the group of adults of higher weight was not observed adverse effect of contaminants, except for exposure to diazinon in esterases. Thus, this study prove clear evidence that it is necessary to take into account in biomonitoring programs, the weight, the tissue and the development, when using tilapia as tested organism.