Cholesteryl hemisuccinate has been incorporated into pea chloroplast thylakoids to investigate the relationship between fluidity and functioning of this membrane system. Levels of sterol which increased the apparent viscosity of the membrane, estimated by fluorescence polarization measurements using the lipophilic probe, 1,6-diphenyl-1,3,5 hexatriene, affected several photosynthetic processes. A decrease in fluidity was accompanied by an inhibition of dark limiting steps associated with electron transfer between photosystems two and one (PSII and PSI) as observed by the oxidation of the primary acceptor of PSII and by electron flow to ferricyanide. Also, treatment with cholesteryl hemisuccinate inhibited the saltinduced rise in chlorophyll fluorescence and changed the ionic conductivity of the membrane as judged by measurements of the decay of the lightinduced proton gradient. The results are discussed in terms of the effect of fluidity changes on the lateral diffusion of plastoquinone and chlorophyll protein complexes in the lipid matrix of the membrane.