Exploratory work was performed on the assimilation of radioactive CO2 in the light by young barley plants. Plants were grown on Hoagland''s nutrient soln. until they were 6-7 inches high. Selected plants were divided into 2 groups (1) those from which primary and secondary roots and hypocotyl were cut ("minus" plants); and (2) entire plants except for secondary roots ("plus" plants). Fresh wts. of each group were about equal. The chamber housing both groups had a rectangular glass frame sandwiched between two 1-inch glass panes; the free vol. was about 1 liter. The chamber was placed in a tank of cool flowing water and illuminated from above, at 18 inches, with 4 GE Projector Spot, 250-watt lamps. A CO2 absorber with NaOH was present to catch gas escaping the spiral immersed in liquid N2. A helium bulb, containing 2-5 p.s.i. of the gas, flushed the spiral containing C14O2 into the chamber. Before and after the expt., H2 flushed other gases out of the tissue. There was a dry ice trap for water. The chamber was filled with H2 and evacuated 5 times to remove CO2 after which the C14O2 was swept in with the help of helium to a 23 inch vacuum and, finally, H2 was admitted to a 5-inch vacuum. The light was turned on and photosynthesis permitted for 60 min. Then the chamber was slowly evacuated, the remaining CO2 being caught in the spiral and NaOH. Six flushings with H2, the final evacuation being to ca. 29 inches, took another hour. The respiration time was thus 2 hrs. Plants from the chamber were placed immediately into liquid N2 and ground to a powder. Treatment of the frozen powder is descr. The crystalline glucose was obtained by evaporating the hydrolyzed sucrose mixture to a syrup, dissolving this syrup with several hundred mg. of carrier glucose in the minimal amt. of 95% alcohol, and adding ether to produce a slight cloudiness. Glucose crystallized out on cooling. The distribution of activity is valid only for the specific set of conditions for photosynthesis given. The activity measurements of the various fractions are tabulated.