The method for the continuous polarographic monitoring of O 2 has been used for systematic studies of the phase and period of the circadian rhythm of photosynthetic O 2 production in individual Acetabularia cells with and without nuclei. The period of the rhythm is highly variable and is modulated by an effective, but imperfect temperature-compensating system which makes the period longer at higher temperatures. Changes in the phase of the free-running rhythm can be effected by single brief dark pulses administered during characteristic portions of the cycle. The equivalence of the dark pulse response curves and temperature coefficients in nucleate and enucleate cells reaffirms the cytoplasm's pronounced capacity for autonomous circadian regulation.