Effects of dieldrin on reproduction of penned hen pheasants (Phasianus colchicus) were measured during 1965 and 1966. Individually caged hens were administered encapsulated dieldrin at weekly intervals. The 1965 treatment levels of 0, 2, or 4 mg per hen per week were administered to birds showing evidence of stress. Of hens surviving, those receiving 4 mg weighed more and laid heavier eggs than the controls. Hatchability of eggs from the 2-mg group was significantly higher for an undetermined reason. Food consumption, egg production, fertility of eggs, and weight gain and survival of chicks were not affected significantly by the treatments. The 1966 treatment levels were 0, 2, 4, or 6 mg per hen per week. The 2- and 4-mg treatments did not appear to influence food consumption and hen weight sufficiently to affect egg production. The 6-mg treatment significantly reduced food consumption, hen weight, and egg production, yet, egg weight, fertility and hatchability of eggs, as well as survival and weight gain of chicks were not reduced significantly. Apparently the 6-mg treatment affected reproduction by lowering the condition of the hens and reducing egg production.