Variations in the susceptibility were found in different Rattus rat­ters populations reared either in areas exposed to the extensive antico­agulant treatment for long time before or in areas its back history en­sured that the anticoagulants were not applied for at least three years at Menofia, Kafr El-Sheikh and Benisuef Governorates. Therefore, series of field and laboratory trials were done. Results showed that rat popula­tions in treated areas exhibited noticeable tolerance to warfarin when applied in the field as compared with those in areas free from any anti­coagulant treatments. Warfarin baits induced 57.5 & 90.0; 81.5 & 95.8 and 87.2 & 93.4% reduction in rat populations in fields treated before or not at the three govemorates.Data of laboratory trials supported those obtained from field experiments, whereas warfarin toxic effect was considerably lower against animals populations collected from agri­cultural lands treated before than those from untreated ones at the three governorates, Le. LD50 values were 90.5,88.0 and 79.4, mg a.i./ kg b.wt. for animals from the first areas in comparison to 79.4, 82.5 and 75.0 mg/kg b.wt. for animals from untreated areas, respectively. A parallel trend was measured against warfarin administration as bleeding time (B.T.) and prothrombin time (P.T.) did not considerably al­ter before and after warfarin treatment in the case of animals exposed for anticoagulant treatment before, while a noticeable prolongation in their values was observed after warfarin treatment in case of animals obtained from unreated areas.