In vivo laboratory experiments on tritium gas (HT) fixation in pine trees were conducted using one-way circuit inhalation apparatus and dwarf pine trees. After 2 hours of exposure to HT, tritium distribution in pine needles, branches, trunk, roots and soil was measured. The specific activity of tritium in pine needles was almost the same as that in the branches and about one fiftieth that in the small roots or soil. To eliminate the contribution of HT oxidation in the soil from the plants, a dwarf pine tree, washed and with its root system sealed, was exposed to HT. Oxidation activity of HT was also determined in in vitro experiments for each part from a dwarf pine tree. Oxidation activity of plant parts above the ground was quite low. In vivo and in vitro experiments were also conducted on dwarf zelkova trees and similar results were obtained.