A fundamental question in systems biology addresses the issue of how flexibility is built into modulatory networks such that they can produce context dependent responses. Here we examine flexibility in the serotonin (5-HT) response system that modulates the cycle frequency (cf) of a rhythmic motor output. We found that depending upon the preparation, the same 5min bath application of 5-HT to the pyloric network of the California spiny lobster, Panulirus interruptus, could produce a significant increase, decrease or no change in steady state cf relative to baseline. Interestingly, the mean circuit output was not significantly different among preparations prior to 5-HT application. We developed pharmacological tools to examine the preparation-to-preparation variability in the components of the 5-HT response system. We found that the 5-HT response system consisted of at least three separable components: A 5-HT_2Pan-like component mediated a rapid decrease followed by a sustained increase in cf. A 5-HT_1Pan-like component produced a small and usually gradual increase in cf. At least one other component associated with an unknown receptor mediated a sustained decrease in cf. The magnitude of the change in cf produced by each component was highly variable, so that when summed they could produce either a net increase, decrease or no change in cf depending upon the preparation. Overall, our research demonstrates that the balance of opposing components of the 5-HT response system determines the direction and magnitude of 5-HT induced change in steady state cf relative to baseline.