Effect of Synethrin on the Toxicity of Dichlorvos in Rats. Hematological, biological and enzymatic effects were investigated in rats treated with DDVP and synethrin. Mutagenicity was also examined. In serological analysis, LDH and ALP were more significantly increased in rats treated with the mixture of DDVP (10mg/kg) and synethrin (250mg/kg) than with either DDVP or synethrin. DDVP alone slightly increased cytochrome P-450 in the liver while synethrin or the mixture decreased it. Lipid peroxidation was significantly increased in the liver when rats were treated with both DDVP and the mixture. Cholinesterase activity was significantly decreased in the liver and serum when treated with DDVP as well as with the mixture. In mutagenicity test, DDVP was shown to be weakly positive to TA 97a, TA 100 and TA 102, but negative to TA 1537. Synethrin showed negative in all strains tested. These results suggest that the mixture of DDVP and synethrin increased the toxicities but not the mutagenicity.