Three primary factors contribute to locomotory speed changes in the pteropod mollusk Clione limacina. (1) An increase in cycle frequency of locomotory appendages is associated with a baseline depolarization and enhancement of postinhibitory rebound in central pattern generator (CPG) interneurons, and a reorganization of the CPG through recruitment of additional interneurons. (2) An increase in the force of appendage movements is generated through enhancement of activity of active motoneurons, recruitment of additional motoneurons and peripheral modulation of swim musculature. (3) Changes in biomechanical aspects of appendage movements are presumably achieved, at least in part, through changes in the activity of motoneurons and swim muscle. All changes associated with non-startle swim acceleration are produced by a serotonergic arousal system that acts at all three levels of the swimming system: CPG interneurons, motoneurons and swim musculature.