The oxidative desulfuration of the three phosphorothionate insecticides--chlorpyrifos, chlorpyrifos-methyl, and leptophos--was studied in rat brain and liver. Hepatic microsomes demonstrated activities of 4-28 nmol/g/min, with male activity 2- to 4-fold higher than female activity. Very low desulfuration activity of all three compounds was observed in both microsomal and crude mitochondrial fractions from brain (3-27 pmol/g/min). There were no sex differences in the brain. Although the liver displayed 140- to 2100-fold greater activity than brain on a wet-weight basis, the brain desulfuration activities of these three compounds as well as those of some previously reported phosphorothionates generally correlate well with the toxicity and may be important in determining the overall acute toxicity levels of phosphorothionate insecticides.