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The kinetic parameters of W. aegyptia venom acetylcholinesterase (AChE) inhibition by procaine and tetracaine hydrochloride were investigated in the present study. Procaine and tetracaine reversibly inhibited the AChE activity in a concentration-dependent manner, the IC50 being about 0.28 and 0.04 mM, respectively. The Michaelis-Menten constant (K-m) for the hydrolysis of acetylthiocholine iodide was found to be 0.051 mM with V-max 10.2 mu mole/min/mg protein. Both K-m and V-max were affected by procaine while only V-max decreased with tetracaine. A Lineweaver-Burk plot and its secondary replot indicated that the nature of the inhibition is of the linear mixed type for procaine which is considered to be a mixture of competitive and noncompetitive types while the inhibition was noncompetitive for tetracaine. The values of K-i(slope) and K-l(intercept were estimated as 0.133 mM and 0.451 mM for procaine and 7.2 x 10(-3) mM for tetracaine, respectively, by the secondary replots of the Lineweaver-Burk plot.