The concerted activity of many neuropeptides has been implicated in the neurohormonal control of specific behaviors and various physiological functions in some invertebrate model systems. What are the functional consequences of this neuropeptide multiplicity? The distinct actions of closely related neuropeptides have been detected in molluscs and insects; however, recent work provides examples of systems in which some of the multiple isoforms may be functionally redundant. Groups of functionally distinct neuropeptides encoded by the same gene can be expressed in different neurons by alternative gene splicing or cell-specific post-translational processing; therefore, as shown recently, they can be targeted for release as 'cocktails' to act on specific sets of muscles or neurons. One prominent role of neuropeptides is to modulate the activity of rhythm-generating circuits, as exemplified by recent research on mollusc neural networks, the crab stomatogastric ganglion, and fly circadian pacemakers.