The optical isomer (+)Physostigmine [(+)Phy] is a very weak anticholinesterase. In a recent report, pretreatment with (+)Phy, at a dose which failed to inhibit acetylcholinesterase (AChE), and atropine provided efficacy against a lethal dose of sarin (SYNAPSE:2, 139, 1988). It was of interest to see whether (+)Phy could protect against soman at a dose which caused only marginal inhibition of the whole blood (WB) AChE in guinea pigs (GPs). (-)Phy (0.15 mg/kg, im) and (+)Phy (10.0 mg/kg, im) produced nearly 70% inhibition of WB AChE at 30 min whereas (+)Phy (0.15 mg/kg, im) caused only marginal inhibition. Groups of guinea pigs (20/group) were dosed, im, with (-)Phy (0.15 mg/kg), (+)Phy (0.15 mg/kg), (+)Phy (10.0 mg/kg) or vehicle (0.5 ml/kg) respectively in one thigh while the mild anticholinergic trihexyphenidyl (THP), 2.0 mg/kg, was injected into the other thigh of 10 animals from each of the respective groups. Thirty min after pretreatment, all animals were challenged with soman (60 micrograms/kg, sc; 2 LD50s); this dose of soman is lethal in unprotected animals. (-)Phy or (+)Phy (10 mg/kg) alone protected nearly 50% from soman lethality, and in combination with THP, all animals survived. In contrast, (+)Phy (0.15 mg/kg; alone or together with THP) was completely ineffective against a 2 LD50 challenge of soman. These data support the hypothesis that protection against soman-induced lethality is related to the degree of carbamylation of the AChE just prior to challenge.