To determine the anticholinesterase properties of huperzine B (Hup B) and compare with tacrine in vitro and in vivo.Spectrophotometry was used to determine ChE activity.Hup B showed much more selective inhibition to acetylcholinesterase (AChE) than tacrine. The IC50 ratios of Hup B and tacrine for butyrylcholinesterase (BuChE): AChE were 65.8 and 0.54, respectively. Hup B ig exhibited higher efficacy on the inhibition of brain AChE than that of tacrine. Tacrine was more effective in the inhibition of serum BuChE in mice with severe concomitant peripheral adverse effects than Hup B. A single ig dose of Hup B produced steady state of AChE inhibition in 4 h.Hup B exhibits higher selectivity and efficacy in the inhibition of AChE, and lower toxicity in mice than tacrine.