The experiment was carried out to determine the effect of a permethrin insecticide on reduced Glutathione (GSH) and Glutathione S-transferase (GST) activity in the liver and serum of poultry birds. Six weeks old poultry birds (broilers) weighing between 400-600 g, divided into four groups of five birds each. The poultry birds were exposed for 30 days to different concentrations (1, 5 and 10%) of a locally manufactured insect powder called ‘Rambo’ which contains 0.6% permethrin. The control group had no insecticide added to their feed. The levels of reduced glutathione in the treated groups were significantly higher than those in the control group (p<0.05) in both the serum and liver of the poultry birds. Liver and serum Glutathione S-transferase activity was significantly lower (p<0.05) in 1, 5 and 10% insecticide concentrations as against the control. The total liver protein was significantly higher at all levels of pesticide exposure compared with the control while serum protein content was significantly higher at 5 and 10% insecticide concentrations. Results of this experiment confirmed the use of GST as a biomarker in ecological evaluation of environmental pollutants.