It has been shown that HI-6 was the most potent oxime so far known in poisoning by sarin, VX , and soman, but its protective effect in tabun poisoning, allegedly due to poor reactivation of inhibited ChE, was much less pronounced. We have found that the thiocholine-like analog of tabun , O-ethyl, N-N- dimethyamino -S-(2-diethylaminoethyl)- thiophosphatemethylsul fomethylate (Ta-S-N+), was very useful in resolving this problem and established the relationship between reactivating and protective effects of PAM-2 Cl, HI-6, and HGG-12 in rats. PAM-2 Cl (protective ratio (PR) = 22.1) and HI-6 (PR = 24.8), combined with atropine, were very effective against Ta-S-N+ poisoning and reactivating inhibited RBC AChE in vitro and rat blood ChE in vivo. The inefficiency of PAM-2 Cl (PR = 1.6) and HI-6 (PR = 2) in tabun poisoning was due to their inadequacy to reactive tabun -inhibited ChEs . The protective effects of HGG-12 in tabun (PR = 2.8) and Ta-S-N+ poisoning (PR = 2.6) were low, and in the absence of any reactivation of inhibited ChEs , have been attributed to its direct pharmacological effects, which were much more potent in the comparison with PAM-2 Cl or HI-6. It is concluded that the reactivation of inhibited ChE is of decisive importance in the efficient protection in poisoning by tabun and other known chemical warfare nerve agents, whereas their direct pharmacological effects are of limited value, allowing survival of animals only against a few LD50s .