Abstract
Resistance level to warfarin was evaluated among 3 colonies of brown rats, Rattus norvegicus, collected in Aomori, Shinjuku and Yokohama. Flourtype baits containing 0.005% (w/w) warfarin were given to them for 12 days. The average survived dose (a.i.) in the Aomori colony was 40.7mg/kg. On the other hand, the lethal days and dose in brown rats were 7 days and 5.7mg/kg in the Shinjuku colony, 4 days and 6.7mg/kg in the Yokohama colony, respectively. Therefore, the Aomori colony was judged as resistant to warfarin, and this is the first case in Japan. On the other hand, the Shinjuku and Yokohama colonies were judged as susceptible to warfarin.
