Lack of neuropathologic consequences of repeated dermal exposure to 2,4-dichlorophenoxyacetic acid in rats.

A 24% aqueous solution of the dimethylamine salt of 2,4-dichlorophenoxyacetic acid (2,4-D amine) was applied to the legs of male Fischer 344 rats 2 hr/day, 5 days/week, for 2 weeks. Because this concentration caused severe skin lesions, a second group of rats was treated similarly with a 12% solution of 2,4-D amine for 3 weeks. The 12% solution caused only mild skin changes. The plasma 2,4-D content, at the end of exposure, was nearly five times greater in the rats exposed to the 24% solution than to the 12% solution (323 vs 66.5 micrograms/ml). The severe skin changes probably facilitated absorption in the rats treated with the 24% solution. Rats treated with either concentration weighed less than controls. Although histologically normal, kidneys of treated rats weighed more than controls. The increased kidney weights were attributed to physiological adaptation due to active excretion of absorbed 2,4-D. Light microscopic examination of tissues, other than skin, revealed no differences between treated and control animals. There were no nervous system pathologic changes although the rats were exposed to sufficient amounts of 2,4-D amine to cause severe skin lesions, decreased body weights, and increased kidney weights.