1. Evidence is presented for the existence of a close relation between the behavior of the contractile vacuole and several phases of the functional activity of Tokophrya infusionum.2. When the process of feeding starts the pulsation rate of the contractile vacuole rises suddenly, remains high during feeding and falls gradually on cessation of the feeding process. It is significant that the pulsation rate remains comparatively high for several hours after the termination of the feeding.3. During reproduction the pulsation rate of the contractile vacuole increases as soon as the endogenous budding starts, rises and remains high until the ciliated embryo has become separated within the brood pouch. This moment is the high peak of the vacuolar activity. The pulsation rate subsequently begins to decrease and falls greatly upon expulsion of the embryo. After expulsion, which lasts several minutes, the pulsation rate of the contractile vacuole drops still lower.4. Unusually accelerated activity of the contractil...