Methoxychlor, a synthetic organochlorine insecticide and structural analog of DDT, was one of several widely used agricultural pesticides selected for bioassay by the National Cancer Institute because of a lack of adequate chronic toxicity data. The suspect state of all DDT-related chemicals was an important additional factor in its selection.
A bioassay for possible carcinogenicity of technical-grade methoxychlor was conducted using Osborne-Mendel rats and B6C3F1 mice. Methoxychlor was administered in the feed, at either of two concentrations, to groups of 50 male and 50 female animals of each species. For each species, 20 animals of each sex were placed on test as controls. The time-weighted average high and low dietary concentrations of methoxychlor were, respectively, 845 and 448 ppm for male rats, 1,385 and 750 ppm for female rats, 3,491 and 1,746 ppm for male mice, and 1,994 and 997 ppm for female mice. After a treatment period of 78 weeks, the rat groups were observed for an additional 34 weeks and the mouse groups for an additional 15 weeks. A dose-related mean group body weight depression was observed in both rats and mice, but no effect on survival was detected.
Under the conditions of this study, methoxychlor was not found to be carcinogenic in Osborne-Mendel rats or B6C3F1 mice of either sex.