Chlamydomonas and Chlorella were grown for 10 days in white light. 955 μw/cm2 blue light (400-500 mμ) or 685 μw/cm2 red light (above 600 mμ). Rates of growth in blue or red light were initially slow, but increased over a period of 5 days until normal growth rates were reestablished. During this adaptation period in blue light, total chlorophyll per volume of algae increased 20% while the chlorophyll a/b ratio decreased. In red light no change was observed in the total amount of chlorophyll or in the chlorophyll a/b ratio. After adaptation to growth in blue light and upon exposure to 14CO2 with either blue or white light for 3 to 10 minutes, 30 to 36% of the total soluble fixed 14C accumulated in glycolate-14C which was the major product. However, with 1 minute experiments, it was shown that phosphate esters of the photosynthetic carbon cycle were labeled before the glycolate. Glycolate accumulation by algae grown in blue light occurred even at low light intensity. After growth of the algae in red light, 14C accumulated in malate, aspartate, glutamate and alanine, whereas glycolate contained less than 3% of the soluble 14C fraction.