Summary Growth and cell wall compositions of Alaska pea seedlings growJl in the dark and light were compared and the following results were obtained. 1. Light effectively inhibited the internode elongation of pea seedlings but, if seedlings grown in the light were transferred to darkness, the internode elopgation was restored. 2. Determination of mechanical properties of the cell wall, as represented by the minimum stress-Ielaxation time, To, showed that in etiolated seedlings the epidermal cell wall of the internodes III and IV was more loosened than that of the basal internodes I and II. This trend was not found in the light-grown seedlings in which the extension of the inner tissue was large in the internode IV as compared with the basal internodes. 3. The amount of cell wall materials and that of noncellulosic and cellulosic polysaccharides per epicotyl was higher in the seedlings grown in the dark than in those grown in the light. However, their amounts per unit length were much higher in the seedlings grown in the light than in those grown in the dark. 4. The cell wall of growing internodes, as compared with nongrowing ones, contained larger amounts of noncellulosic than of cellulosic polysaccharides. 5. In general, the amounts of uronic acids and neutral sugars in the pectin fraction, hemicelluloses 1 and 2 were higher in the cell wall of dark-grown seedlings than in that of light-grown ones, per internode as well as per mg cell wall. 6. In the hemicellulose 1, in the cell wall of dark-grown seedlings the amount of arabinose was higher in the third and fourth internode, as compared with the first and second internode. In the seedlings grown both in the dark and in the light there was a general trend that the amount of galactose was higher and that of xylose was lower in the third and fourth internode than in the first and second one. 7. The composition of neutral sugars in the hemicellulose 2 varied relatively little with the age of internodes and with the growth conditions. It appeared, however, that in light-grown seedlings the amount of xylose was reduced and that of glucose was elevated in the third and fourth internode than in the first and second one. From these results it was concluded that the metabolic turnover of sugars in the polysaccharides, particularly neutral sugars present in the pectin fraction, is closely related with the extension of epicotyls. Light caused the thickening of the cell wall by increasing the amounts of both cellulose and nonrellulosie polysaceharides, particularly increasing the amount of xylans in the latter.