The effects of chronic aluminum (Al) administration on the deposition of the metal and on the receptor binding characteristics of the Ml muscarinic acetylcholine receptors (MlAChR) were studied in selected rat brain areas. Animals were injected intraperitoneally with an AlCl3 solution of 1.0 mg/ml/100 g of body weight for 5 weeks, 5 days a week. Al accumulation was detected by solochrome azurine histochemistry in the brain, where the metal could be visualized in capillaries, endothelial cells and surrounding brain tissues. Changes in the binding properties of the MlAChR after chronic Al treatment were determined with the use of selective and nonselective muscarinic antagonists. Significantly decreased number of maximal MlAChR binding sites (Bmax) as measured by the equilibrium binding of [3H]pirenzepine, were detected in all of the brain areas examined. While the nonselective antagonist [3H] (-)QNB displayed a generally decreased Bmax, value, it reached the level of significance only in the striatum. These results provide a further indication that chronic Al treatment results in the accumulation of Al in the brain and consequently affects the cholinergic neurotransmission.