The conversion rate of tritiated water (HTO) in plants' leaves to organically bound tritium (OBT) in their edible parts in the open air was obtained for several vegetables (komatsuna, radish and cherry tomato) during a chronic tritiated hydrogen (HT) release experiment at Chalk River in 1994. During the experiment, HT gas was continuously released to the atmosphere at the cultivated site for 12 days, and HTO and OBT concentrations in cultivated plants were measured. For plants' leaves, the conversion rate of HTO to OBT was about 0.2 (% hr−1), but it varied with their growth stage. A chamber experiment was also carried out in laboratory for comparison. The chamber experiment results showed that the conversion rate under artificial lights was half that in the field experiment.