There are reciprocal stimulations of Cl influx by K and Na, and of K and Na influx by Cl, in the light in Hydrodictyon africanum. The component of the K influx which stimulates, and is stimulated by, Cl, is independent of the ouabainsensitive mechanism for K influx also found in H. africanum. The concentration dependence of the cation effects on Cl influx and on the Cl-stimulated portion of their own influxes are similar. The stimulation with K saturates at about 0.3 mM K; that with Na saturates at about 2 mM Na. The Cl-dependent portions of the K and Na influxes have similar responses to changes in photo-synthetic metabolism (far-red illumination, CDMU, and CCCP) as does the light-stimulated Cl influx. This suggests that Cl influx, and the Cl-stimulated portions of K and Na influxes are both dependent on photosystem 2 of photosynthesis, and are less sensitive to the uncoupler CCCP than is 14CO2 fixation or the K-Na pump. It would thus appear that the Cl-dependent portions of the K and Na influxes in the light are linked to the cation-stimulated portion of the Cl influx. There is no very great change in the electrical component of the inwardly directed passive driving force on cations under conditions in which Cl is being pumped compared with those under which it is not. It is not clear whether such increase in this driving force as do occur could account quantitatively for the increase in the cation influxes associated with Cl transport, or whether chemical coupling must be invoked. In addition to the Cl-stimulated portions of the cation influxes, there are also light-stimulated portions of K and Na influx which are independent of Cl, not associated with the cation regulating mechanism, and which seem to have a similar linkage to photosynthesis as does the Cl-K-Na pump. Since the light-stimulated portion of the K efflux appears to be similar to this portion of the K influx, these Cl-independent light-stimulated portions of K and Na influxes are tentatively related to light-induced changes in cation permeability.