This study revises the mechanisms of ciliary locomotion and demonstrates muscular contribution to locomotion rate in Lymnaea stagnalis. L. stagnalis sticks to the substratum by the foot sole and moves smoothly with no visible contractions of the foot. A ciliated epithelium covering the sole is underlain by smooth muscle cells containing giant mitochondria. It is shown here that slow (basal) locomotor activity (measured as the flow rate of physiological saline over isolated sole) appears spontaneously or is induced by 10(-8)-10(-7) M 5-HT. 5-HT (10(-7)-10(-4) M) facilitates locomotor activity dose-dependently, and KCN (an inhibitor of mitochondrial respiration) decreases these effects to the basal level. 5-HT and KCN have no effect on the frequency of ciliary beat (stroboscopic measurements), and blockers of anaerobic glycolysis inhibit ciliary motility. Under anaerobic conditions locomotion of a snail is slow and insensitive to 5-HT in contrast to that in aerobic environments. It is concluded that glycolysis supplies energy to ciliated cells and respiration to sole muscle cells; 5-HT stimulates ciliary beating in an all-or-none fashion and muscular waves in a dose-dependent manner; cilia provide slow (basal) gliding, and locomotory rate up to 80% above the basal level is determined by muscular waves.