Ciliary reversal occurs in response to electrical and chemical stimuli in specimens of Paramecium caudatum in which ciliary beat has been completely inhibited by external application of nickel ions. The mechanism underlying ciliary reversal appears, therefore, to differ from that of ciliary beat. The cessation of ciliary beat has no effect on the intracellular potential of Paramecium. However, depolarizing action potentials are associated with ciliary reversals in paramecia, treated with nickel, without ciliary beat. Thus, membrane depolarization in this species seems specifically concerned with the ciliary reversal, and not with ciliary beat.