Fenitrothion (0.10 mg/kg body weight) was fed to pigeons for 1, 2, 4 and 6 weeks to study the hematological responses as induced by this organophosphate. Gross alterations of hematological indices were noticed in experimental birds treated for two or more weeks. Progressive reduction of total count of RBC, hemoglobin content, ESR, hematocrit and mean corpuscular hemoglobin concentration followed by subsequent increase of the total count of white blood cells, mean corpuscular volume and mean corpuscular hemoglobin were noticed. Both the total cellular count and weight of spleen significantly decreased. Bleeding and clotting time became conspicuously prolonged in the experimental birds. Marked heterophilia and eosinophilia together with lymphopenia, monocytopenia and basopenia were noticed. Gross cellular distortion of particularly the heterophil and also of eosinophil was observed. The specific cytotoxic effect on the heterophils can be used for toxicity monitoring studies. The pattern of response remained similar in all the experimental groups, though the reactions intensified as the experiments progressed. The results suggest possible hemorrhage-induced anemia, stress effect and immunosuppression by fenitrothion. The hematological response following this insecticide became consistent from two weeks of treatment, thereby indicating the potential of using two week exposure period to monitor pesticide toxicity by using hematological indices.