Perisynaptic Schwann cells (PSCs) play critical roles in regulating and stabilizing nerve terminals at the mammalian neuromuscular junction (NMJ). Howe¹er, although these functions are likely regulated by the synaptic properties, the interactions of PSCs with the synaptic elements are not known. Therefore, our goal was to study the interactions between mammalian PSCs in situ and the presynaptic terminals using changes in intracellular Ca 2ϩ as an indicator of cell activity. Motor nerve stimulation induced an increase in intracellular Ca 2ϩ in PSCs, and this increase ÿas greatly reduced when transmitter release was blocked. Furthermore, local application of acetylcholine induced Ca 2ϩ responses that were blocked by the muscarinic antagonist atropine and mimicked by the muscarinic agonist muscarine. The nicotinic antagonist ␣-bungarotoxin had no effect on Ca 2ϩ responses induced by acetylcholine. Local application of the cotransmitter ATP induced Ca 2ϩ responses that were unaffected by the P2 antagonist suramin, whereas local application of adenosine induced Ca 2ϩ responses that were greatly reduced by the A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT). However, the presence of the A1 antagonist in the perfusate did not block responses induced by ATP. Ca 2ϩ responses evoked by stimulation of the motor nerve were reduced in the presence of CPT, whereas atropine almost completely abolished them. Ca 2ϩ responses were further reduced when both antagonists were present simultaneously. Hence, PSCs at the mammalian NMJ respond to the release of neurotransmitter induced by stimulation of the motor nerve through the activation of muscarinic and adenosine A1 receptors.