Methamidophos (O,S-dimethyl phosphorothioamidate) causes polyneuropathy in man and hens. However, experiments in the hen show that lower doses of methamidophos either protect from or promote the neuropathy caused by certain organophosphates. The initiation of neuropathy as well as protection from neuropathy are thought to be related to neuropathy target esterase (NTE), whereas promotion is likely to be due to interactions with another unknown target. Methamidophos is a racemate and we report studies with its resolved optical isomers, aimed at elucidating which isomer is responsible for the described effects. The time-course of acetylcholinesterase (AChE) and NTE activity in nervous tissues of hens after inhibition by single doses of either isomer showed that after D-(+) methamidophos (25 mg/kg PO) peak inhibition of both enzymes was achieved within 24 h (80-90%). However, after L-(-) methamidophos (15 mg/kg PO), peak inhibition (80-90%) was obtained within 24 h for AChE, whereas similar NTE inhibition (120 mg/kg PO) was observed only 4 days after dosing. The minimal neuropathic doses of D-(+) and L-(-) methamidophos were 60 and 120 mg/kg PO, respectively, and correlated with > 80% NTE inhibition in nervous tissues. OPIDP initiation by either isomer was slightly promoted by phenylmethanesulfonyl fluoride (120 mg/kg SC). D-(+) Methamidophos (25 mg/kg PO) partially protected from dibutyl dichlorovinyl-phosphate (DBDCVP) neuropathy (up to 0.8 mg/kg SC). This effect correlated with about 70% NTE inhibition. L-(-) Methamidophos (15 or 60 mg/kg PO) did not protect from DBDCVP neuropathy (0.2-0.8 mg/kg SC).