The effect of the muscarinic agonist McN-A-343 [4-(m-chloro-phenylcarbamoyloxy)-2-butynyltrimethylammonium chloride] on the electrical-field stimulation-evoked overflow of tritium from rabbit isolated pulmonary artery preloaded with (-)-[3H]noradrenaline (3H-NA) was studied. At 10-6-3 x 10-3 M, McN-A-343 caused a temporary enhancement of the 3H overflow. At 10-4 and 3 x 10-4 M, McN-A-343 initially enhanced the 3H overflow and thereafter inhibited it. Cocaine at 10-6-3 x 10-5 M also enhanced the 3H overflow evoked by stimulation, but to a lesser degree than McN-A-343. At 10-4 and 3 x 10-4 M, cocaine solely reduced the 3H overflow. The spontaneous outflow of 3H from pulmonary artery preloaded with 3H-NA was not altered by McN-A-343 (10-4 M). The mean overflow from pulmonary artery preloaded with 3H-NA during stimulation consisted of 3H-NA (28%), [3H]3,4-dihydroxy-phenylglycol (3H-DOPEG) (10%), [3H]3,4-dihydroxymandelic acid (3H-DOMA) (4%), [3H]O-methylated plus deaminated compounds (3H-OMDA) (52%), and [3H]normetanephrine (3H-NMN) (6%). Initially, McN-A-343 only decreased 3H-DOPEG. Subsequently, the amounts of 3H-NA and 3H-NMN were also reduced, with a corresponding rise in 3H-DOMA and 3H-OMDA. Cocaine (3 x 10-5 M), desmethylimipramine (10-7 M), atropine (3 x 10-7 M), N-methylatropine (10-5 M), hexamethonium (3 x 10-5 M), and suprofen (3 x 10-5 M) did not prevent the initial enhancement induced by McN-A-343 (10-4 M). Cocaine (3 x 10-5 M) and desmethylimipramine (10-7 M) abolished the inhibitory effect of McN-A-343 (10-4 M), while atropine (3 x 10-7 M), N-methylatropine (10-5 M), hexamethonium (3 x 10-5 M) and suprofen (3 x 10-3 M) did not. McN-A-343, cocaine, and desmethylimipramine reduced the neuronal uptake of 3H-NA (10-3 M) by rabbit aorta. The order of inhibitory potency was desmethylimipramine > cocaine ≫ McN-A-343. It is concluded that McN-A-343 enhances the stimulation-evoked 3H overflow by inhibiting the neuronal membrane pump and by facilitating transmitter release. The enhancement is not mediated by either muscarine or nicotine receptors and is prostaglandin-independent. McN-A-343 inhibits the stimulation-evoked 3H overflow by entering into the adrenergic nerve endings via the membrane amine pump and causing an adrenergic neurone blockade at an intracellular site.