The experiments were done to investigate the presence and subtype of functionally presynaptic muscarinic receptors in cholinergic nerves of the guinea pig urinary bladder. Bladder strips were incubated with 3H-choline and superfused with Tyrode's solution containing eserine. Secreted 3H-acetylcholine was separated from 3H-choline. The electrically evoked 3H-acetylcholine secretion increased with the stimulation frequency. 3H-Acetylcholine secretion was enhanced by muscarinic antagonists, was depressed by carbachol and by alpha adrenoceptor agonists but was not influenced by drugs acting at beta adrenoceptors or purinoceptors. The rank order for the enhancing effect of muscarinic antagonist EC50 values was propantheline < atropine < methylatropine < N-desethyloxybutynin < UH-AH 37 < benzhexol < AQ-RA 741 < 4-DAMP < procyclidine < emepronium < secoverine < oxybutynin < tropicamide < promethazine < himbacine < hexahydrosiladifenidol < methoctramine = pirenzepine < dicyclomine < AF-DX 116, and the EC50 values correlated best with constants for the M4/m4 muscarinic receptor subtype. The enhancing effect of atropine was counteracted by carbachol; the effects of atropine and emepronium were not additive. The 3H-acetylcholine secretion was also enhanced by forskolin, 3-isobutyl-1-methylxanthine, 8-bromo cyclic AMP and dibutyryl cyclic AMP. The combined effects of atropine and forskolin were additive. These results suggest that the 3H-acetylcholine secretion in the guinea pig urinary bladder is regulated by a presynaptic muscarinic autoreceptor of the M4 subtype that is not coupled to adenylate cyclase.