Acceleration of swimming velocity was induced by the transfer of Paramecium cells to solutions containing various multivalent cations, Fe³⁺, Ca²⁺, Mg²⁺, Tris⁺, and others. The monovalent cations, K⁺, Rb⁺, Li⁺ and Na⁺, however, suppressed the acceleration induced by the multivalent cations. Effects of Ca²⁺ and K⁺ on swimming velocity were antagonistic, as the Ca²⁺ concentration increased the K⁺ concentration acted to suppress the Ca²⁺ induced acceleration. As both the Ca²⁺ and K⁺ concentrations were low, this antag-onistic relation could be represented by the concentration ratio, [K⁺]/[Ca²⁺]^1/2.
Acceleration of the swimming velocity took place when cells were transferred to a solution with a lower concentration ratio than that of the adapting solution used. Time courses of decreasing velocity after acceleration were examined at various concentrations of Ca²⁺ and K⁺, and for various temperatures. Change in membrane potential as measured with a microelectrode, was not consistently related to the change in swimming velocity. These results are discussed in relation to the driving force for the influx of Ca²⁺.