Acid soil is one of the problem soils. Plant growth is usually significantly inhibited by strong acidity, which may result in a high concentration of Al, excessive content of heavy metals such as solubilized Fe and Mn, poor nutrition such as that due to P deficiency, the elution of bases such as Ca, etc. In order to apply environmental remediation or crop production to acidic soil where such combined stress is present, we searched for acid-tolerant plants and made physiologic analysis of their response to acidity and the adaptive function of the plants. In order to select for acid-tolerant plants, 22 kinds of plants were grown in a sandy soil (pH3.9) to which sulfuric acid had been added; and the amount of growth of these plants was measured. As a result, kenaf (Hibiscus cannabinus) and buckwheat (Fagopyrum esculentum) comparatively showed excellent growth in the acidic soil. On the other hand, red clover (Trifolium preatense) and alfalfa (Medicago sativa) did not grow at all in it. Next, we examined the change in the pH of the rhizosphere of plants grown under the acidic condition. As a result, kenaf and buckwheat, which were able to grow in acid soil, showed an elevated pH of their rhizosphere. On the other hand, the intolerant red clover showed no increase in the pH of its rhizosphere. Using the kenaf, we collected substances from the roots of plants that had been grown in solution culture under the strongly acidic condition, and tried to identify the substances involved in the neutralization of the rhizosphere. As a result, we found that ammonia was secreted from the kenaf roots under the acidic condition. Also, the pH of the solution culture rose as the amount of the ammonia secretion increased. Thus we showed ammonia to be one of the neutralizing substances. When the relationship between the pH of the solution culture and the amount of ammonia secretion from the roots was examined, ammonia was not secreted under the neutral to weak acidic condition. But because the amount of the ammonia secretion rapidly increased in the vicinity of pH3, we presume that ammonia secretion was supposedly induced by the low pH. From these results, we confirmed that the acid-tolerant plant can neutralize the low pH of the rhizosphere by secreting ammonia from its roots under the acidic condition, thus indicating ammonia secretion to be one of the resistance mechanisms resulting in acid tolerance in plants. Acknowledgement: This study was supported by the 21st Century Center of Excellence (COE) Program of the Ministry of Education, Culture, Sports, Science, and Technology, Japan.