β‐Amyloid (Aβ) deposits in diffuse and compact senile plaques in the brain are one of the defining histopathological features of Alzheimer's disease (AD). Preventing Aβ deposition is a goal of drug therapy for AD, because excessive amounts of Aβ may be toxic to neurons. In preclinical studies, activation of the muscarinic M1 receptor subtype inhibited Aβ secretion from cultured cells. To determine whether a similar sequence occurs in human beings, we administered the selective M1 agonist AF102B to 19 AD patients and measured total Aβ (Aβ_total) levels in cerebrospinal fluid (CSF) before and during treatment. Aβ_total levels in CSF decreased in 14 patients by 22%, increased in 3 patients, and were unchanged in 2 patients; the overall decrease in the group as a whole was statistically significant. To test the specificity of the M1 effect, we also measured the relative changes in Aβ_total levels in CSF during treatments in separate sets of AD patients with the acetylcholinesterase inhibitor physostigmine or the anti‐inflammatory drug hydroxychloroquine. CSF Aβ_total levels did not change significantly in the 9 AD patients in the physostigmine protocol or in the 10 AD patients in the hydroxychloroquine study. These data provide evidence that the activation of M1 receptors reduces Aβ levels in the CSF of AD patients. If this effect also occurs in brain, M1 agonists may have long‐term therapeutic benefits by lowering amyloid in AD. Ann Neurol 2000;48:913–918