Chlorella pyrenoidosa were allowed to photosynthesize for short periods of time in the presence of (14)CO(2) and HTO. Analysis of tritium and (14)C labeling of photosynthetic intermediate compounds showed that the T/(14)C ratio of glycolic acid was comparable to that of intermediate compounds of the photosynthetic carbon reduction cycle when photosynthesis was performed in nearly 100% oxygen and only slightly higher under steady-state conditions. It is concluded that formation of labeled glycolic acid as a consequence of its proposed hydrogen transport role in photosynthesis is quantitatively of limited importance compared to the net synthesis of glycolic acid from CO(2).