The light-saturated rates of electron transport of the photoreactions, H 2 O → DCIP and H 2 O→ methyl viologen in isolated pea thylakoids are inhibited in the presence of cupric chloride with an I 50 value of 7.5 μM. This inhibition is more or less equal for both the light-limited and the light-saturated rates suggesting a decrease in the concentration of active PS II reaction centres. Chlorophyll variable fluorescence ( F v ) at room temperature in the presence and absence of DCMU is inhibited by copper; the apparent I 50 value being 25 μM and 30% of F v remains insensitive to copper at a concentration which inhibits electron transport completely. When the insensitive part of Fy is subtracted out, the I 50 value becomes 7.5 μM. The rate of electron transport in H 2 O → silicomolybdate (+ DCMU) photoreaction is inhibited by copper in a manner similar to the inhibition of F v . The inhibition of manganese → DCIP and manganese → silicomolybdate (+ DCMU) rates of electron transport in heat-treated thylakoids is similar to that of H 2 O → DCIP and H 2 O → silicomolybdate (+ DCMU), respectively. These results are interpreted as that copper inhibits the primary photochemistry of only one fraction of PS II centres (α centres/B-type centres) while the other fraction (β centres/non-B-type centres) is insensitive to copper. The strongest evidence of copper inhibition of the PS II reaction centre (RC) is that copper inhibits chlorophyll fluorescence in the isolated PS II RC which contains two polypeptides, Dl and D2, one cytochrome b -559 apoprotein, 4–5 Chi a , one P-680, two pheophytin and one β-carotene molecules but no Q A or Q B . Copper binds to the isolated RC in a stoichiometry of 0.8 copper per RC.