1. The small cardioactive peptides (SCPs) are potent modulatory neuropeptides in Aplysia. Buccal ganglia neurons B1 and B2 are the largest neurons that exhibit SCP-like immunoreactivity. High-pressure liquid chromatography (HPLC)-bioassay and in vivo radiolabeling procedures confirm that these neurons contain and synthesize very large quantities of SCPA and SCPB. 2. Both B1 and B2 innervate the gut. HPLC-bioassay measurements indicate that the SCPs are present throughout the anterior sections of the gut. SCP-like immunoreactivity was largely confined to fibers and varicosities in the gut, although occasional immunoreactive enteric neurons were also observed. The purpose of this study was to determine the physiological roles of B1 and B2 and to what extent these roles are mediated by release of the SCPs. 3. Low-frequency tonic stimulation of B1 led to an increase in peristaltic contractions in a relatively distal portion of the gut. This action could be mimicked by superfusion of the same portion of the gut with very low concentrations of the SCPs. 4. B2 produced discrete contractions of the anterior portions of the gut only when fired in bursts. These actions could not be reproduced by superfusion with the SCPs and may be mediated by ACh. 5. B1 and/or B2 are active during the swallowing cycle of each feeding movement, which suggests that these effects on the gut are likely to occur during feeding. Thus the SCPs play a major role in the central regulation of gut motility.