In Gonyaulax polyedra it is possible to measure overt rhythms in luminescence, photosynthesis, and cell division. A common endogenous oscillator appears to control all three processes. The question of concern here is the manner in which the information regarding period and phase is transduced from oscillator to overt rhythm. In the rhythm of photosynthetic capacity, the path of electron flow through systems I and II appears not to be the site of transduction, since there is no rhythm in cells in flashing light, in the Hill reaction, or in sensitivity to such specific inhibitors as dichlorophenyl dimethyl urea (DCMU), and carbonyl cyanide m-chlorophenylhydrazone (CCmP). The probable site of control is in the Calvin cycle, since the activity of the first enzyme in this cycle, ribulose diphosphate dicarboxylase, varies with the phase of the cells from which the enzyme is derived. The low activity of this enzyme in crude extracts from cells in the night phase in continuous light can be overcome by increasing the concentration of bicarbonate in the reaction mixture. In vivo also, increasing the concentration of bicarbonate decreases the amplitude of the rhythm markedly. The activity of mixtures of extracts prepared during the day and the night phase is intermediate between that of either enzyme preparation alone, suggesting that the differences in activity are not caused by the presence of activators or inhibitors. The activity of ribulose diphosphate carboxylase is reduced by high temperature and by the presence of parachloromercuribenzonate (pCMB) and both adenosine triphosphate (ATP) and adenosine diphosphate (ADP), but differences in sensitivity to inhibitors between "day" and "night" enzyme preparations are not observed. The concentration of ATP extractable from cells does not vary with the phase of the rhythm.Transduction in the luminescent rhythm appears to be via changes in the mechanism by which luminescence is stimulated in vivo, since it is possible to obtain large and almost equal amounts of light from cells throughout the rhythmic cycle by the addition of acid. Eliciting luminescence in this way appears to bypass the normal mechanism of stimulation. This conclusion is strengthened by the observation that the inhibitory effect of light on cell luminescence is also eliminated when acid is used.