1. The photosynthetic activity of Chlorella pyrenoidosa, expressed by the ratio Photosynthesis highest in young chlorophyll cultures and declines from the beginning of the growth. 2. By cultivation under different external conditions of light and temperature, cells of different photosynthetic activity can be formed. 3. The decline of the photosynthetic activity cannot be explained by accumulation of photosynthetic products. 4. A specific effect of magnesium deficiency on the photosynthetic activity of young cells, as has been found by Fleischer, could not be reproduced. 5. The decline of the photosynthetic activity cannot have been caused by inactivation of the chlorophyll, but must be ascribed to the activity of the Blackman reaction. 6. The changes in the rate of photosynthesis and of catalase activity do not correspond, so it seems unlikely that the Blackman reaction and the splitting of added hydrogen peroxide should proceed by means of the same mechanism. 7. If the decline of the total photosynthetic capacity of an aging culture is caused by nitrate deficiency, it can be raised by nitrate addition. This can be achieved equally well in light as in darkness if only the cells contain a sufficient amount of carbohydrates. If no carbohydrates are present the rate of photosynthesis can only be increased in light when new carbohydrates are photosynthesized. 8. The increase of the photosynthetic activity by nitrate addition is established independently of the chlorophyll. 9. The growth and chlorophyll formation of Chlorella in glucose containing media are equally good in darkness as in light. The absence of light during the growth does not have an effect on the photosynthetic power of the cells. 10. The fact whether the cells have built up their carbohydrates of carbon dioxide or of glucose does not affect the photosynthetic activity.