The physiological response of field grown grapevine (Vitis vinifera L. cv. Marzemino) plants to grapevine leafroll-associated virus (GLRaV-1) was studied. Changes in photosynthetic pigments and in photosynthetic activity were investigated. GLRaV-1 considerably decreased the leaf net photosynthetic rate (Pn), stomatal conductance (gs) and the transpiration rate (E) in grapevine leaves, and also strongly reduced pigments, soluble proteins, ribulose-1,5- bisphosphate carboxylase (RuBPC) and nitrate reductase activity. In isolated thylakoids, the virus strongly inhibited whole-chain and photosystem (PS) II activity, while PSI activity was only marginally inhibited. The artificial exogenous electron donors diphenyl carbazide, manganese chloride (MnCl2) and hydroxylamine (NH2OH) did not restore lost PSII activity to virus-infected leaves. Chlorophyll fluorescence suggested that the inhibition of primary light reactions was a major effect of virus infection. Immunological studies showed that D1 protein levels of the PSII reaction centre were significantly lower in virus-infected leaves. It is concluded that the decreases in photosynthetic pigments and photosynthetic activities caused by the virus strongly impair photosynthesis in Marzemino grapevine plants.