O-ethyl-O'-(2-diisopropylaminoethyl) methylphosphonite (QL), an intermediate in the formation of ethyl S-2-diisopropylaminoethyl methylphosphonothiolate, was evaluated for neurotoxicity in the adult hen. Birds were given a single oral dose of QL ranging from 635 to 6080 mg kg-1. 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 LD50 in hens is 1186 mg kg-1. 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 QL-treated hens at dose levels greater than or equal to 635 mg kg-1, oral dose. These findings indicate neural damage tendencies following QL exposure.