Abstract In the present work we studied the effect of Cu deficiency on spinach chloroplasts. We found that in spinach the electron transport was inhibited as reported previously for sugar beet (Droppa, M., Terry, N. and Horváth, G. (1984) Proc. Natl. Acad. Sci. USA (1984) 81, 2369–2373). The breakpoint of the Arrhenius plot of the whole electron-transport activity was shifted from +6°C to +12°C in Cu-deficient chloroplasts. A similar effect could be observed with a spin-labelled probe, when the rotational correlation time was plotted vs. the reciprocal temperatures. This indicates that the membrane fluidity might be changed by Cu deficiency. The lipid/protein ratios were similar in both control and deficient chloroplasts. On the other hand, the saturated/unsaturated ratio of phosphatidylcholine (PC), phosphatidylglycerol (PG) and sulpholipids (SL) was increased but that of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) decreased. We conclude that Cu deficiency does not change the entire membrane fluidity but rather the lipid composition of the microenvironment of some electron-transport components. The inhibition of Photosystem II electron transport in Cu-deficient chloroplasts was characterized by thermoluminescence and 2-dimensional gel electrophoresis. It was found that Cu deficiency shifted the main peak of the glow curve from +18°C to +8°C, similar to that of DCMU-poisoned chloroplasts. Two apoproteins of the 29 kDa polypeptide disappeared in Cu-deficient chloroplasts which indicates that this polypeptide has a regulatory role in ensuring the normal electron flow between Q A and Q B.