Acetylcholinesterase (AChE) activities in different brain parts were determined quantitatively in rats treated with huperzine A, soman, and huperzine A followed by soman, using histochemical and biochemical methods. Following soman intoxication ( 1.2 x LD50, i.m.), AChE activity was decreased to 30-80% of control values depending on the brain structure. The most sensitive area was the frontal cortex and the most relatively resistant was ncl.ruber. Huperzine A treatment only caused a change in AChE activity varying from 70 to 100% of control values. In rats pretreated with huperzine A and intoxicated with soman, AChE activity was significantly higher than that observed after soman. In these animals, survival of rats pretreated with huperzine was observed while the mortality of unpretreated animals was near to 80%. The results suggest that huperzine A is good candidate for further study for clinical use as a prophylactic drug against nerve agent poisoning.