The dissociation constants have been determined and compared for a series of reversible, noncovalent inhibitors of eel acetylcholinesterase that are structurally related to the very potent inhibitor, 1,2,3,4-tetrahydro-9-aminoacridine (THA). It is concluded that there exists on the enzyme protein, closely adjacent to the anionic subsite, a conformationally flexible, hydrophobic area which tends readily to assume a near planar form. The dimensions of this area are unknown, but it is adequate in size to fully accomodate THA. It is this area, acting conjointly with the adjacent anionic subsite, which provides the attraction for THA and related inhibitors. Uv absorbance maxima and pKa vlaues are reported for many of the compounds.