The median lethal dose of orally administered norbormide for wild Rattus norvegicus found to be 9·0 mg./kg. of body weight and the LD 95 about 17·0 mg./kg.

In tests in which various concentrations of norbormide or zine phosphide were added to the food of individually caged wild rats, mortality increased with concentration of poison, though more slowly with norbormide than with zine phosphide. The mortality that occurred among rats offered a choice between unpoisoned food and the same food with added norbormide or zinc phosphide indicates that in control treatments in the field the optimum concentration of norbormide in bait would be about 0·;5% and that this might be expected to give results comparable with those obtainable with 2·;5% or 5·0% zine phosphide. Other methods of estimating suitable field strengths indicate that concentrations of norbormide higher than 0·;5% may be preferable.

Some animals that survived exposure to a choice of plain food and the same Food poisoned with norbormide or zine phosphide at field concentrations avoided mating lethal amounts by reacting to the taste of the poison. Others learned to use the taste of the food, not that of the poison as a cue and later avoided eating the food when it contained no poison.

When either poison was presented to rats in the more palatable of two foods in the choice situation mortality was relatively high. Some of the surviving animals subsequently rejected the more palatable food in preference to the normally less palatable alternative.