Marked agevariation in the susceptibility of wild Norway rats to acute poisoning with tt-naphthyl thiourea has been found, With suckling rats about 7 times as resistant as adults. This resistance decreased with increasing body weight, levelling off for adults at an LD50 between 6 and 8 mg./kg. body wt. The difference found between adult d* d31 and 9 9 was within the limits of error and is therefore not considered significant. Adult animals of other spp. also varied markedly in susceptibility to acaite;A(FJTIP poisoning. Norway rats, dogs, and mice were killed by amts. less than a hundred mg./kg. Alexandrine rats, guinea pigs, and cats required several hundred mg./kg., while chickens and monkeys survived doses of several g./kg. Severe pulmonary edema was found only in Norway rats, dogs, mice, and cats.