The stomatogastric nervous system (STNS) of decapod crustaceans has long been used to study the modulation of small neural circuits. Profiles in the sheath of the nerves and ganglia of the STNS, which contain only dense-core vesicles, have been described in electron microscopical studies (Friend [1976] . These profiles resemble those found in neurohemal organs and suggest the presence of neurohemal release zones in the STNS. To map these putative neurohemal release zones, a combination of two antibodies was used in the present study. A synapsin antibody recognizing vesicle proteins of clear vesicles was combined with a synaptotagmin antibody recognizing vesicle proteins of clear and dense-core vesicles. Exclusive synaptotagmin-like staining, therefore, indicated the regions with only dense-core vesicles. Such a staining was found in a mesh in the perineural sheath of nerves in the STNS of all three species investigated. In the crayfish Cherax destructor and the lobster Homarus americanus, the stained mesh was located in the sheath of nerves connecting all four ganglia of the STNS, whereas in the crab Cancer pagurus it was found on different nerves, which are more directly exposed to the hemolymph in this species. Exclusive synaptotagmin-like staining was also found in a putative neurohemal release zone in the sheath of the circumoesophageal connectives and the postoesophageal commissure in C. destructor. These data suggest that an important source of modulation of the networks and the muscles of the stomach is a compartmentalized release of neurohormones from zones in the STNS.