Grapevine plants (Vitis vinifera L. cv. Silvaner) were cultivated under shaded conditions in the absence of UV radiation in a greenhouse, and subsequently placed outdoors under filters transmitting natural radiation, or screening out the UV-B (280 to 315 nm), or screening out the UV-A (315 to 400 nm) and the UV-B spectral range. All conditions decreased maximum chlorophyll fluorescence (F(M)) and increased minimum chlorophyll fluorescence (F(0)) from dark-adapted leaves; however, with increasing UV, F(M) quenching was stimulated but increases in F(0) were reduced. The F(V)/F(M) ratio (where F(V)=F(M)-F(0)) was clearly reduced by visible radiation (VIS): UV-B caused a moderate extra-reduction in F(V)/F(M). Exposure of leaves (V. vinifera L. cv. Bacchus) to UV or VIS lamps quenched the F(M) to similar extents; further, UV-B doses comparable to the field, quenched F(0). A model was developed to describe how natural radiation intensities affect PS II and thereby change leaf fluorescence. Fitting theory to experiment was successful when the same F(M) yield for UV- and VIS-inactivated PS II was assumed, and for lower F(0) yields of UV- than for VIS-inactivated PS II. It is deduced, that natural UV can produce inactivated PS II exhibiting relatively high F(V)/F(M). The presence of UV-inactivated PS II is difficult to detect by measuring F(V)/F(M) in leaves. Hence, relative concentrations of intact PS II during outdoor exposure were derived from F(M). These concentrations, but not F(V)/F(M), correlated reasonably well with CO(2) gas exchange measurements. Consequently, PS II inhibition by natural UV could be a main factor for UV inhibition of photosynthesis.