The rhythmically active pyloric and gastric mill motor patterns in the stomatogastric ganglion of the crab, Cancer borealis, are influenced by modulatory projection neurons whose somata are located primarily in the other ganglia of the stomatogastric nervous system. One of these projection neurons exhibits substance P-like immunolabeling. However, bath application of substance P does not influence these motor patterns. To determine whether a different peptide is responsible for the substance P-like immunolabeling, we studied the presence and physiological effects of the locustatachykinins and the leucokinins, two families of tachykinin-like peptides originally identified in insect nervous systems. Locustatachykinin-like immunolabeling has the same distribution in the stomatogastric nervous system as substance P-like immunolabeling and colocalizes with it in the majority of immunopositive structures. Preincubation of locustatachykinin antibody with substance P, and preincubation of substance P antibody with locustatachykinin, blocks subsequent immunolabeling in the stomatogastric nervous system. In contrast, we found no leucokinin-like immunolabeling in this system. Bath application to the stomatogastric ganglion of individual locustatachykinins or leucokinins excited the pyloric rhythm in a state-dependent manner. Each peptide family had distinct effects on the pyloric rhythm. Thus, both of these tachykinin-like peptide families are likely related to native neuropeptides that influence the pyloric rhythm. Furthermore, a member of the locustatachykinin family is likely to be the source of the previously identified substance P-like immunoreactivity in the stomatogastric nervous system.