Wild Norway rats (Rattus norvegicus) from several habitats were tested for their susceptibility and resistance to warfarin. Animals were fed 0.005% warfarin in ground oatmeal for periods varying from 1 to 12 days. Rats having no prior exposure to anticoagulants were shown to be susceptible, none having survived a 6-day feeding period. Resistant rats were found on two farms where anticoagulant materials had been used intensively for about 20 years.