The purpose of the present study was to evaluate methanesulfonyl fluoride (MSF), a very long-acting CNS-selective acetylcholinesterase (AChE) inhibitor, as a palliative treatment for senile dementia of the Alzheimer type (SDAT). In experiment I, MSF (0.03-0.18 mg/kg) was administered orally to 10 normal volunteers to measure toxicity and establish dose/response function in erythrocyte AChE. MSF produced a dose-response function of %inhibition = (40)(Log10[MSF mg/kg] + 51.7) with no toxicity at these doses. Experiment II was a 16-week double-blind, placebo-controlled study of the safety and efficacy of MSF in doses of up to 0.18 mg/kg given three times per week in 5 men and 10 women (60-82 years), with Mini-Mental State Examination (MMSE) scores of 9-24, who had SDAT. MSF produced a mean of 89.5% inhibition of erythrocyte AChE in patients and improved cognitive performance as measured by the MMSE, Alzheimer Disease Assessment Scale-Cognitive Subscale (ADAS-COG), Global Deterioration Scale, and the Clinical Interview Based Impression of Change (CIBIC). Most of the improvement on the ADAS-COG was maintained 8 weeks after ending MSF. No patients left the study because of drug-related adverse events and there were no toxic effects. MSF may be a safe and effective palliative treatment for SDAT and further clinical trials in larger groups of patients are warranted.