Heavy water vapor release experiments were carried out in a greenhouse using deuterium as a substitute for tritium and uptake and loss kinetics of D2O in leaves and formation, translocation and retention of organically bound deuterium (OBD) in rice and soybean were investigated. Rate constants of D2O uptake in leaves of rice plant and soybean in the daytime release were 2.4 and 3.0 hr−1, respectively, and 5-4 times higher than those in the nighttime release. Rate constants of D2O loss in leaves after daytime release were about twice those after the nighttime release. The half time of D2O loss was 0.6–0.7 hr for leaves of rice plant and soybean. After D2O release, OBD concentration in unhulled rice and soybean increased with time until 4 –5 days of the experiments and then decreased with time and the extent of decrease was remarkable in soybean pea.