The bispyridinium oxime HI-6, 1-((((4-amino-carbonyl)pyridinio)methoxy) methyl)-2-(hydroxyimino)methyl)pyridinium dichloride monohydrate, combined with atropine is an effective treatment for soman (pinacolyl methylphosphonofluoridate) poisoning but is relatively ineffective against tabun (ethyl N-dimethyl phosphoroamidocyanidate) poisoning in mice. This contrasts with those results obtained using the bispyridinium oxime obidoxime[1,1'-(oxy bis(methylene)) bis(4-(hydroxyimino)methyl) pyridinium dibromide]. The purpose of this study was to investigate the efficacy of the combination of HI-6 and obidoxime plus atropine against poisoning by tabun and soman in mice. The combination of ineffective single doses of obidoxime (5 or 10 mg/kg) and HI-6 (25 or 50 mg/kg) improved the treatment of tabun poisoning over either oxime alone. Combinations employing higher concentrations of obidoxime (25 or 50 mg/kg) and HI-6 (100 or 200 mg/kg) resulted in significant toxicity in the absence of organophosphate poisoning. Against soman poisoning the addition of obidoxime to HI-6 did not attenuate the efficacy of HI-6. The half-life of elimination and peak serum concentrations of HI-6 and obidoxime were not altered following administration of the combined injection. Reactivation of tabun-inhibited acetylcholinesterase was found consistently in the diaphragm but not in the brain. Using response surface methods it was possible to estimate the optimal therapy against soman and tabun poisoning (74.5 mg/kg HI-6 + 31.9 mg obidoxime against 1052 microns/kg obidoxime against 390 microns/kg challenge of soman).(ABSTRACT TRUNCATED AT 250 WORDS)