The process of photosynthetic energy conversion in Chlorella pyrenoidosa was investigated by simultaneous measurement of transient and steady-state rates of O
2 evolution and fluorescence. 1. 1. Alternation or superimposition of light 1 and light 2 illumination induces both fast and slow changes in fluorescence and rate of O
2 evolution. The fast changes are ascribed to changes in conditions of the reaction centers in the context of the Hill-Bendall
1 model and the kinetic analysis of Eley and Myers
2. The slow changes are interpreted as adaptations to the intensity and wavelength of illumination. The adaptive mechanism is described in terms of slow variation in fraction (α) of total absorbed quanta delivered to System 2. At low intensities, the calculated value of α for cells adapted to light 2 illumination (light 2 state) is approx. 0.9 of α for cells adapted to light 1 illumination (light 1 state). 2. 2. An increase in fluorescence yield was found to accompany the decrease in O
2 yield at the onset of light saturation with either light 1 or light 2 excitation. Variation in α is proposed to account for the differences between the maximum fluorescence yield observed in steady-state conditions and the 1.5 times higher maximum yield observed in transient conditions or in cells inhibited by 3(3,4-dichlorophenyl)-1,1-dimethylurea. Variation in α can also explain the observation of a higher rate of fluorescence emission with light 1 excitation than with light 2 excitation for a given steady-state rate of O
2 evolution. 3. 3. A model for energy conversion by System 2 is proposed to account for our observations. The model proposes competitive dissipation of absorbed energy by photochemical trapping at reaction centers and by fluorescence and radiationless de-excitation from both the pigment bed and reaction centers of System 2. © 1969.