1. The average lethal dose of fluoracetamide for wild R. norvegicus was found to be 13 mg./kg. of body weight and the LD95, about 43 mg./kg.
2. Wild rats offered bait contatining 0·5, 1 and 2% fluoracetamide, after a period of conditioning, seemed to find it palatable and, on the average, ingested the equivalent of 18–100 LD50's. Warning symptoms appeared in about an hour or longer and included short convulsive spasms.
3. The speed of onset of warning symptoms leading to cessation of feeding was studied in albino rats dosed with the equivalent of 2 LD50's of sodium fluoracetate, zinc phosphide or fluoracetamide. Feeding ceased soonest in the case of sodium fluroacetate. Zinc phosphide gave very variable results, but it is thought that if wild rats had been used, feeding would have stopped nearly as soon as with sodium fluoracetate.
4. It is considered that field trials to compare the efficiency of 1% flouracetamide and 2·5% zinc phosphide, both with ‘prebaiting’, would be well justified.
5. Fluoracetamide at 2% may also prove to be a good alternative to 0·25% sodium fluoracetate as a ‘direct’ poison for controlling rats in sewers.
We are indebted to Miss Y. Larthe and Miss E. J. Taylor who took part in the bioassay and in one of the experiments with the marked rats.