In order to clarifyin vivo relationships between the mammalian metabolism and toxicity of temephos, an organophosphate insecticide, activities of acetylcholinesterase and hepatic mixed function oxidases (MFO) were evaluated in adult female Sprague-Dawley rats after acute and subchronic temephos exposures. After acute and the last subchronic exposure, animals were exsanguinated at 2, 4, 6, 8, 12, 24, 36 and 48 hr. Plasma was collected for temephos extraction and subsequent analysis by high performance liquid chromatography. Red blood cell acetylcholinesterase (RBC AChE) activity was assayed radiometrically immediately after blood collection. Hexobarbital sleeping times after temephos treatments were measured four hr after the acute (1 day) and last subchronic (5 days) exposures to assess MFO activity. After acute and subchronic exposures, temephos elimination was monoexponential with half-lives of 7 and 24 hr, respectively. After acute temephos exposure, RBC AChE was inhibited by 67% at 4 hr and 47% after 48 hr. After subchronic temephos exposures, RBC AChE inhibition was 100% throughout the 48-hr period with presence of cholinergic signs. Plasma temephos concentrations were inversely correlated with RBC AChE inhibition (r=−0.92). Duration of hexobarbital sleeping times increased significantly after acute and subchronic treatments with high temephos doses. Plasma temephos concentrations were directly correlated with hexobarbital sleeping times (r=0.87). The results indicate that temephos effects on MFO were similar to many other organophosphates. Increased plasma elimination half-lives indicated MFO saturation and partition of temephos into peripheral tissues at high doses. RBC AChE inhibition was consistent with other organophosphates that are metabolized to active metabolites. MFO saturation resulted in optimum metabolite production and maximum RBC AChE inhibition after subchronic exposures.