UV-B treatment of thylakoid membranes or fractions derived from them accelerates the first rise (Fo---Fi phase) of variable fluorescence and slows down the second (Fi---Fm) rise. This can be observed for all membrane fractions but to different extents. The grana fraction is affected more by the UV-B treatment than are the intact thylakoids and the stroma lamella fraction. An exogenous electron acceptor, 2,6-dichloro-p-benzoquinone, inhibits both the initial and second rises of the fluorescence induction kinetics including the changes induced by UV-B. However, other electron acceptors, such as 2,5-dimethyl-p-benzoquinone and methyl viologen quench only the second rise of the curve. Oxygen evolution from grana-derived membranes is more sensitive to UV-B treatment than oxygen evolution from stroma lamella-derived membranes.