The photosynthetic performance of synchronously grown Chlamydomonas reinhardtii alternated rhythmically during the cell cycle. The activity of the "CO(2) concentrating mechanism" including the ability to accumulate CO(2) internally and the activity of carbonic anhydrase peaked after 6 to 9 hours of light and reached minimum after 6 to 9 hours of dark. Consequently, the apparent photosynthetic affinity to extracellular CO(2) alternated rhythmically. At the end of the dark period the cells behaved as if they were adapted to high CO(2) even though they were continuously aerated with air. Results from experiments in which the light or dark periods were extended bear on the interaction between the internal (cell cycle or biological clock) and the external (light) signal. The observed rhythmical alterations in photosynthetic V(max) may result from changes in PSII activity. The latter may be partly explained by the capacity for phosphorylation of thylakoid proteins, which reached maximum after 9 hours of light and decreased toward the dark period.