The circadian pacemaking system resides at the cellular level and consists of: (1) input system conveying ambient signals such as illumination to the pacemaker; (2) the circadian pacemaker, performing autonomous oscillation with a circadian period; and (3) physiological functions (output system), the activities of which oscillate with the circadian period being driven by the pacemaker. The ciliate protozoan, Paramecium has all the basic components of animal functions residing within a single cell, including the reception of external stimuli, and an excitable membrane to transduce and transfer the signals to effectors, such as motor organelles, which express responsiveness to changes in environmental conditions. Four circadian phenomena have been so far described in Paramecium: cell division, locomotor behavior, mating reaction and resting membrane potential. The latter three have been studied recently by three groups in Japan, and these are briefly reviewed here. These studies in Paramecium have well documented the roles of second messengers in the regulation of ciliary movement and membrane potential, both of which are considered to be tightly coupled. Paramecium, therefore, provides an excellent model to pursue upstream the second messenger pathways to the pacemaker from observable circadian Physiological phenomena.