The light-dependent chloride absorption in Vallisneria leaves was studied in its relation to the respiration-dependent chloride absorption, and to photosynthesis. Light-dependent chloride absorption begins at full rate some five minutes after the light is turned on; it ends within two minutes after the light is turned off (Fig. 6). Light saturation is reached at a much lower light intensity than with photosynthesis (Fig. 7). The action spectra of chloride absorption and photosynthesis are identical (Fig. 8), so chlorophyll is involved in light-dependent chloride absorption. Carbon dioxide has no instantaneous influence on chloride absorption in the light, which proves the absence of a direct relationship between chloride absorption and carbon assimilation. Bicarbonate ions inhibit chloride absorption in the light as well as in the dark, but chloride ions do not inhibit photosynthetic bicarbonate assimilation. Carbon monoxide inhibits the respiration-dependent chloride absorption only; it has no influence on light-dependent chloride absorption or photosynthesis (Chapter 6). The same probably holds for oxygen withdrawal (Chapter 7). Cyanide inhibits chloride absorption instantaneously and fully reversibly. The inhibition of photosynthesis increases gradually when the leaf tissue is in the light until a stationary level is reached; in the dark the reaction with cyanide which causes the inhibition does not take place at all (Fig. 20). The inhibition is gradually reversible. In Chapter 9 the photosynthetic assimilation of carbon dioxide and bicarbonate has been treated; a correction of an earlier paper on photosynthesis in Vallisneria has been included there. The experimental data are discussed in Chapter 10.