Kepone® (decachlorooctahydro-1,3,4-metheno-2H-cyclobuta[cd] pentalen-2-one), a chlorinated organic insecticide, was included in the diet of a mixed lot of laboratory mice totaling 66 pairs at 0, 10, 17.5, 25, 30, and 37.5 ppm for 1 month before and 5 months after mating, to determine the effects on reproductive success. In a 2nd test, a total of 48 pairs of mice of the pure inbred BALB/c strain was fed 0, and 10 ppm for 1 month before mating and 4 months after mating. This was followed by a 3rd test involving 24 control pairs and 36 pairs on 5 ppm Kepone. Reproduction was reduced at all feeding levels, and was expressed by a decrease in the size and number of litters. The greater the concentration of insecticide in the diet, the greater was the effect. A 4th experiment with progeny of the animals used in the 3rd test also showed decreased reproductive success.