To test the effect of a single pass through the liver, 'Isolan' was injected directly into the hepatic portal system (intestinal vein) in one group of rats and into the general circulation (femoral vein) in another group. Simlar investigations were conducted with two organic phosphorus compounds-dichlorvos, a direct inhibitor of cholinesterase, and parathion, an indirect inhibitor. It is well known that prathion is converted to the highly toxic paraoxon in the liver, but it is presumably in the liver also that it is eventually detoxified. Even though other techniques will have to be used to explain fully the pattern of toxicity of 'Isolan' when given by different routes, the technique presented in this report is a useful and relatively simple procedure for learning the net effect of the liver on the metabolism of chemicals in intact laboratory animals.