The muscarinic toxin MT7 is the most selective ligand for the muscarinic M(1) receptors. Previous studies have shown that the toxin interacts with the antagonist-receptor complex and slows the antagonist dissociation rate, possibly by binding to an allosteric site and impeding the access to and egress from the orthosteric binding pocket. In the present study, we investigated the action of MT7 on agonist-occupied receptors in functional and radioligand binding assays of the cloned human muscarinic M(1) receptor expressed in Chinese hamster ovary cells. In time-course experiments, the addition of MT7 rapidly blocked the acetylcholine-stimulated guanosine-5'-O-(3-[(35)S]thio)triphosphate binding to membrane G proteins. Similarly, in acetylcholine-treated cells MT7 completely stopped the agonist-stimulated [(3)H]inositol phosphate accumulation. In dissociation experiments using membranes pre-equilibrated with [(3)H]acetylcholine, the addition of MT7 increased the rate of radioligand dissociation. The data indicate that MT7, while partially stabilizing the antagonist-receptor complex, effectively destabilizes the agonist-occupied muscarinic M(1) receptors.