In the present study, close spatial relationships between peripheral serotoninergic nerve elements and the musculature of the body in planaria Polycelis tenuis, Schmidtea mediterranea, and Girardia tigrina have been demonstrated using immunocytochemical and histochemical methods and a confocal laser scanning microscopy. Such a localization of serotoninergic neurons and their fibers indicates an important role of serotonin in the regulation of the muscle function in planaria. Investigation of the mechanisms of muscle contraction in planaria have shown that depolarization caused by high concentration of potassium ions (15–100 mM) and serotonin (10–4 –10–9 M) induced contractions of isolated muscle fibers of Procerodes littoralis. Dihydropyridine calcium channel blockers, nicardipine, nitrendipine, and nifedipine, inhibited contractions of muscles induced by potassium and serotonin suggesting the dependence of the muscle contraction on the extracellular calcium. Thapsigargin and cyclopiazonic acid decreased the number of muscle cells contracting in response to potassium ions, but did not influence the induction of the contractions by serotonin. Thus, the muscle contraction caused by serotonin did not depend on the intracellular calcium. The results provide evidence of the presence of different types of receptors and ion channels mediating the muscle contraction in planaria.