Concentrations of 0, 1 and 3 ppm endrin in dry duck mash were fed to mallards (Anas platyrhynchos) starting in December, and health and reproduction were measured the following spring and summer. One male fed 3 ppm endrin died with 2.0 ppm endrin (wet weight), a diagnostically lethal level, in its brain. Birds fed 1 ppm endrin reproduced as well as, if not better than, controls. Birds fed 1 ppm endrin had significantly greater hatching success of fertile eggs than did those fed 0 or 3 ppm, and their clutches hatched significantly earlier than did those of birds fed 3 ppm. Mallards fed 3 ppm endrin appeared to reproduce more poorly than controls, but this finding must be regarded with caution because the results of statistical tests often were not significant. Endrin accumulated in eggs to a mean of 1.1 and 2.9 ppm (wet weight) when fed to hens at 1 and 3 ppm. The concentration of endrin in the cacasses of adults was similar to that in eggs, but the concentration in the fat of adults was about 4 to 7 times higher than in eggs.