O-ethyl-O'-(2-diisopropylaminoethyl) methylphosphonite (QL), an intermediate in the formation of ethyl S-2diisopropylaminoethyl methylphosphanothiolate, was evaluated for neurotoxicity in the adult hen. Birds were given a single oral dose of QL ranging from 635 to 6080 mg kg-'. The QL-treated hens were observed for up to 24 h after dosing for acute toxicologic effects and over a 24 d post-dose period for evaluation of delayed neurotoxicity. The oral LD,, in hens is 1186 mg kg-'. Neurologic dysfunction, as evidenced by motor incapacitation, was observed at 6 days and thereafter after treatment with QL. Neurotoxic esterase (NTE) activity was not inhibited at 24 h after exposure to compound. Neural damage, multifocal in nature, was noted in the peripheral nervous system of QLtreated hens at dose levels 3 635 mg kg-', oral dose. These findings indicate neural damage tendencies following QL exposure. EXPERIMENTAL Chemical source and purity Tri-ecresyl phosphate (TOCP) was obtained from Pfaltz and Bauer, Stamford, CT. Paraoxon (0,Odiethyl-O-(Cnitrophenyl) phosphate) was procured from Aldridge Chemical Co., Milwaukee, WI. Mipafox (8 N'diisopropyl phosphorodiamidic fluoride) was provided by Bayer Chemical Co., Leverkusen, West Germany. O-ethvl-0'-( 2-diisopropvlaminoethyl ) methylphosphonite (QL), purity > 95'70, was provided by the Organic Chemistry Division,