The effects of in vitro ageing and salinity of the reaction medium on the primary photochemistry of photosystem II and the pattern of energy distribution within the photochemical apparatus of thylakoids isolated from Pisum sativum and Spinacia oleracea are described. Analyses of the low temperature induction curves of fluorescence emission at 695 and 735 nm and of low temperature absorption and fluorescence emission spectra were used to examine these processes. In vitro ageing over short periods reduced the photochemical activity and changed the energy distribution within thylakoids of P. sativum, but had little effect on thylakoids of S. oleracea. A synergistic effect of in vitro ageing and salinity of the reaction medium was observed for P. sativum thylakoids. Ageing effects could be minimized by addition of 100 mM NaCl to the resuspension medium. Changes in NaCl concentration in the reaction medium produced large and similar changes in the primary photochemical functioning of thylakoids from P. sativum and S. oleracea, which could be attributed mainly to the cation species, Na$; however, experiments using mannitol to produce osmotic stress indicated some small osmotically induced changes in photofunction of the thylakoids. Optimal primary photochemical activity of photosystem II, for both species, was observed with 200 mM NaCl. Cation-induced regulation of exciton distribution appears to be facilitated by controlling the degree of energy coupling between the light-harvesting chlorophyll a/b complex and the two photosystems, I and II, and not by regulation of coupling between photosystems I and II.