We have studied the dynamic properties of acetylcholinesterase dimer from Torpedo californica liganded with tacrine (AChE-THA) in solution using molecular dynamics. The simulation reveals fluctuations in the width of the primary channel to the active site that are large enough to admit substrates. Alternative entries to the active site through the side walls of the gorge have been detected in a number of structures. This suggests that transport of solvent molecules participating in catalysis can occur across the porous wall, contributing to the efficiency of the enzyme.