A series of 2-alkoxy-2, 8-diazaspiro[4.5]decane-1, 3-diones and related compounds were synthesized and tested for muscarinic receptor binding affinity using [³H]pirenzepine and [³H]oxotremorine M as ligands. They were also evaluated for agonistic activities in the guinea pig ileum assay. 2-Methoxy-2, 8-diazaspiro[4.5]decane-1, 3-dione (1i) was found to be a relatively M1 selective agonist. It reversed CO₂-induced impairment of passive avoidance response with long duration of action, but also displayed peripheral effects at low doses. To minimize these side effects, we proposed the idea of conjugation of 1i with a muscarinic antagonist. The carbamate linked conjugate (1u) of 1i with methylatropine was therefore examined.