Trace amounts of elemental tritium (HT) were released continuously to the surface atmosphere at a site at Chalk River Laboratories, Canada over the 12-day period 1994 July 27 to August 8. The test area consisted of four agricultural plots, each 5 m square. One plot was left in its natural state, and the other three were cultivated. Cherry tomatoes, radishes and edible Chinese mustard (Komatsuna in Japanese) were grown in the three cultivated fields. After the HT gas reached the surface of the field, some portion of it was converted into the water form (HTO) mainly by microorganisms in the soil. Then, plant absorbs HTO from both soil and air. Photosynthesized organic material in plant contains tritium,also. The main purpose of this research is to analyze the variation with time of tissue free-water tritium (TFWT) and organically bound tritium (OBT) in several plant species during the HT release. The specific activity of HTO in plants increased rapidly after the HT exposure. The HTO concentration in plant leaves almost attained its maximum value in about 10 days, even though there were several precipitation events, which decreased the soil HTO concentration. The specific activity of OBT in plants increased gradually after the start of the exposure. The OBT concentration in plants did not attain steady state over the whole exposure period of 12 days. The OBT/HTO ratio in plants increased gradually during the release. The ratio in Komatsuna leaves changed from 0.06 to 0.24 between 2 and 11 days. The production rates of OBT in plants are fitted to numerical equations.