Changes in the membrane-associated Ca2+ concentration and viscosity of the plasmatic membrane of P- and R-cells segmented ganglia of medicinal leech upon a variation of extracellular serotonin concentration were investigated. It is shown that the transmission of rhythmic excitation from P- to R-cells is due to the formation of the serotonin-dependent synapse and affects the distribution of the membrane-associated Ca2+ and intracellular processes controlling the viscosity of the membrane.