Summary. The frog neuromuscular junction is sensitive to nitric oxide (NO), since exogenously applied NO reduces the release of transmitter by presynaptic terminals and the size of ATP-induced Ca2+responses in perisynaptic Schwann cells. This study aimed at determining whether an NO synthase (NOS) is present at the neuromuscular junction, notably in perisynaptic Schwann cells, the glial cells at this synapse. The NADPH-diaphorase (NADPH-d) histochemical technique revealed the presence of NOS in cell bodies and presumed processes of perisynaptic Schwann cells. Incubation with NOS inhibitors, NG-nitro-L-arginine methyl ester or NG-monomethyl-L-arginine-acetate, abolished the NADPH-d staining. Moreover, L-arginine, the precursor of NO, impeded the blockade by NOS inhibitors, establishing the NOS specificity of NADPH-d staining in frog tissue. The pattern of labelling with a polyclonal antibody against the neuronal form of NOS was similar to the NADPH-d staining, also suggesting the presence of a neuronal NOS in perisynaptic Schwann cells. Using electron microscopy, the NOS immunostaining was found at the membrane and occasionally in the cytoplasm of perisynaptic Schwann cells and was not detected in the nerve terminal or muscle. There was no enzymatic or immunocytochemical labelling of NOS 6 days after denervation. It is concluded that NOS is present in frog perisynaptic Schwann cells. The presence of this endogenous NOS suggests that NO may act as a diffusible glial messenger to modulate synaptic activity and synapse formation at the neuromuscular junction.