AbstractPerfusion of14C‐(ring)‐parathion or14C‐(ring)‐paraoxon with blood through isolated, intact rat livers resulted in the rapid degradation of these insecticides. Degradation was negligible in the absence of rat liver (controls), thus demonstrating the capacity of the liverper seto effectively degrade these compounds. Of the total radiocarbon recovered after liver perfusion with [14C]parathion, 33 % could be attributed to unchanged [14C]parathion (similarly distributed between the liver and the blood) while 67.9 % was degraded to water soluble compounds and 2.5% was converted to organic soluble paraoxon and traces of p‐nitrophenol. Nearly all of the [14C]paraoxon, however, was degraded by the intact rat liver, resulting in water soluble products that amounted to 98.5% of the total radiocarbon recovered. Unexplained losses of radiocarbon with the perfusion apparatus used were lower in the presence of rat liver which degraded the insecticides to more water soluble compounds. The water soluble degradation products produced from [14C]parathion and [14C]paraoxon were non‐toxic to mosquito larvae(Aedes aegyptiL.). These ring‐labelled products were found to be conjugatedp‐nito‐phenol. Nearly all of the water soluble radiocarbon was located in the perfused blood, while only small amounts (1.8 to 3.0% of recovered) were excreted via the bile or were associated with the liver tissue (1.3 to 1.