In the original Karnovsky and Roots' method for the localization of acetylcholinesterase (AChE), thiocholine reduces the ferricyanide and cupric ions of this medium competitively, giving simultaneously cupric (Koelle's precipitate) as histochemical products. We modified the method in order to promote the true Karnovsky's reaction, and to slow down the secondary Koelle's reaction by increasing the concentration of the ferricyanide ion from 0.5 mM to 5.0 mM and by decreasing the concentration of the cupric ion from 3.0 mM to 2.5 mM. The cupric ion, complexed with 5 mM sodium citrate in the original method, was further stabilized by the use of 0.1 M citrate buffer in order to prevent the interaction of cupric ion with increased ferricyanide. In order to suppress completely the residual Koelle's precipitate, we used acetylthiocholine chloride as a substrate, instead of acetylthiocholine iodide. The chloride salt of cuprous thiocholine is soluble, contrary to the iodide salt. In addition, the pH of the medium was lowered from 6.0 to 5.0 to avoid artefactual nuclear staining, appearing at a pH beyond 5.5. In this modified medium, Karnovsky's cupric ferrocyanide becomes the sole precipitate at the enzymatic site and this provides fine localization of acetylcholinesterase activity.