We have previously demonstrated the presence in Alzheimer's disease brain of an endogenous inhibitor of choline acetyltransferase activity. Selected properties of these compounds were investigated. There appear to be two distinct classes of inhibitor present, both phosphomonoesters and nonphosphorylated substances. They are not proteins, pass through 500 mm dialyses membranes and are not lipoidal. There are both different sensitivities of individual control cytosotic activity to inhibition and differences in intrinsic inhibitory activity present in individual Alzheimer's disease brain samples. There is a competitive type of inhibition with respect to acetyl CoA as substrate and a noncompetitive type with respect to choline as substrate.