Deltamethrin [( S]-alpha-cyano-3-phenoxybenzyl-cis-(1R,3R)-3-(2,2-dibromovinyl+ ++) (2,2-dimethyl-cyclopropane-carboxylate], is the most potent insecticide known at the present time. But it is also one of the most toxic pyrethroids for vertebrates. The toxicity study of deltamethrin was performed on mice and rats, and on anaesthetized dogs, the administration route being either oral or intravenous. The oral LD50 of deltamethrin suspended in 10% gum-arabic solution was 5.54 +/- 1.29 g/kg p.o. in male mice and 3.45 +/- 1.27 g/kg p.o. in female mice. In rats and anaesthetized dogs, deltamethrin at high concentrations by the oral route engendered neither mortality nor signs of intoxication. When dissolved in glycerol formal and given intravenously, the LD50 of deltamethrin was as low as 3.44 +/- 0.67 mg/kg in anaesthetized dogs. Values obtained for the toxicity of deltamethrin varied not only with the animal species and sex involved, but also with the administration routes and solvents used. Administered orally, it was 100 times less toxic when suspended in gum-arabic solution than dissolved in oil or organic solvent. Whatever the animal species, sex, administration routes and solvents employed, the poisoning symptoms of deltamethrin are identical, i.e. salivation, ataxia and choreoathetotic movements with rolling convulsions, appearing within 7 h after administration. No cellular alteration was detectable by means of optical microscopy of excised organs.