Knowledge about the relationships between the intensity of salinity of rooting medium and dry matter production may contribute to the improvement of rice yield under saline conditions. A rice cultivar Koshihikari was grown in pots with soils under submerged soil conditions. The stagnant water was salinized with NaCl from 31, 45 and 59 days after sowing, and for each of those, the level of salinity was increased every 6 days to maturity at three different rates of increase. Electrical conductivity (EC) of stagnant water was measured. Final dry weight of above ground parts and panicles decreased as initiation of salinization took place earlier and the rate of increase of EC was higher. Reduction in panicle dry weight was correlated with the occurrenceof white heads. The values of EC at which plants died increased with the plant development and the rate of increase of EC, while these depended mainly on the rate of increase of EC but not on the time of salinization. In the plants which survived after heading, the final dry weight decreased in proportion to accumulated EC (sum of EC values for each day until harvest). Therefore, except for strong salinity where plants may die before heading, accumulated EC should be abe to predict dry matter production under salinity as an integrated indicator of level and duration of salinity.塩性環境における稲作改善のために，イネの乾物生産と根域の塩性程度との関係を知ることは重要である．水稲品種コシヒカリを用い，土壌を詰めたポットで湛水栽培し，湛水に塩化ナトリウム（塩）を添加することによって塩ストレスを与えた．塩添加は播種後31，45および59日目から開始され，それぞれ６日ごとに３段階の異なる速度で塩を添加するとともに，湛水の電気伝導度（EC）を測定した．地上部および穂の最終的な乾物重は，塩添加開始が早いほど，EC の増加速度が大であるほど低下した．そして，穂の乾物重は白穂の発生率が高いほど低下した．植物体が枯死したときの EC 値もまた，塩添加の開始時期が早いほど速度が大であるほど大きかったが，この EC 値は主にEC 増加速度に依存しており，塩添加開始時期の影響は小さかった．一方，出穂した植物で乾物重は積算 EC 値（毎日の EC の値を植物体採取日まで足した値）に対して直線的に低下した．以上より，出穂する前に枯死するような強い塩性環境を除けば，積算 EC 値は塩ストレスの強さと期間を総合した指標で，イネの最終的な乾物生産をよく予測すると考えられた．