Effects of auxin to dwarf mutants of rice plants (Oryza sativa L.) were examined by the measurement of length in coleoptile and by the increment of angle between laminae and sheaths of excised leaves in rice plant. In order to investigate the effect of auxin to coleoptile of dwarf rice plants, seeds of these varieties were soaked in auxin solution and distilled water, and sown on 0.6% agar. After growth in the dark for 6 days at 300℃ the coleoptile length was measured. From measurement of angles between laminae and sheaths in excised leaves floated on distilled water, it is suggested that level of endogenous auxin was higher in normal than in dwarf rice plants. There were differences among the dwarf types in response to auxin. Namely, Daikoku, Bunketsu-to and Waisei-shirasasa responded more to IAA (Indole acetic acid) than other dWarf types in both treaments described above. Bunketsu-to and Waisei-shirasasa showed especially a great response to NAA (Napthalen acetic acid) in both treatments. Daikoku showed a great response to NAA in length of coleoptile, but did not in lamina joint test. Other dwarf types and normal type are small in response to auxin including both treatments. Bunketsu-to showed a striking elongation of a main culm by the decapitation of tiller and by the treatment with NAA spray. From these results, it is supposed that the action.of dwarf gene in Bunketsu-to is closely connected with development of tiller and with auxin metabolism.