We used intracellular recording to investigate how muscarinic acetylcholine receptors and the serine kinase signal transduction cascade are involved in regulating transmitter release in the neuromuscular synapses of the levator auris longus muscle from adult rats. Experiments with M1 and M2 selective blockers show that these subtypes of muscarinic receptors were involved in enhancing and inhibiting acetylcholine (ACh) release, respectively. Because the unselective muscarinic blocker atropine considerably increased release, the overall presynaptic muscarinic mechanism seemed to moderate ACh secretion in normal conditions. This muscarinic function did not change when more ACh was released (high external Ca2+) or when there was more ACh in the cleft (fasciculin II). However, when release was low (high external Mg2+ or low external Ca2+) or when there was less ACh in the cleft (when acetylcholinesterase was added, AChE), the response of M1 and M2 receptors to endogenously released ACh shifted to optimize release, thus producing a net potentiation of the Mg2+-depressed level. Protein kinase A (PKA) (but not protein kinase C, PKC) has a constitutive role in promoting a component of normal release because when it is inhibited with N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, 2 HCl, release diminishes. The imbalance of the muscarinic acetylcholine receptors (mAChRs) (with the selective block of M1 or M2) inverts the kinase function. PKC can then tonically stimulate transmitter release, whereas PKA is uncoupled. The muscarinic function can be explained by an increased M1-mediated PKC activity-dependent release and a decreased M2-mediated PKA activity-dependent release. In the presence of high external Mg2+ or low Ca2+, or when AChE is added, both mAChRs may potentiate release through an M2-mediated PKC mechanism and an M1-mediated mechanism downstream of the PKC.