Radioactive starch, glucose and fructose have been prepared from tobacco leaves after assimilation of C14O2. The apparatus used for photosynthesis consisted of a shallow Perspex leaf chamber connected to a closed gas system, in which C14O2 was generated from BaC14O2. Six leaves, area 14 to 18 sq. dm. when exposed to bright sunlight with an initial CO2 concentration of 8 to 10 per cent., assimilated 3.35 g. of C14O2 in 8 to 10 hours. At least 80 per cent. of the C14O2 supplied appeared in the leaves as starch and sugar and over 80 per cent. of the radioactivity was accounted for in these carbohydrates. The specific activity per m. atom of carbon of the isolated products was 85 to 90 per cent. of that of the C14O2. Small amounts of radioactive carbon were also incorporated in the leaf protein and in the celluose, hemicellulose and polyuronides.