We aim to determine how the actions of circulating hormones modify the influence of modulatory projection neurons on a small motor circuit. To this end we are studying how the hormone crustacean cardioactive peptide (CCAP) alters the gastric mill (chewing) rhythm (GMR) elicited by the projection neurons MCN1 and CPN2 in the stomatogastric ganglion (STG) of the crab Cancer borealis. Selective stimulation of MCN1 and co-activation of MCN1 and CPN2 elicit distinct GMRs. CCAP application directly excites a subset of gastric mill neurons but it does not activate the GMR. However, CCAP (threshold <0.1 nM) lowers the minimum MCN1 firing rate needed for GMR activation. When the MCN1 firing rate is held constant, CCAP appears to slow the rhythm by prolonging the protractor phase of the MCN1-elicited GMR (Kirby & Nusbaum, SFN 2003). Here we show that CCAP also alters the MCN1/CPN2-elicited GMR and it does so in a manner comparable to its actions on the MCN1-elicited GMR. We also show that there is not a strict corre...