Inhibition of photosynthesis was followed during autumn and early winter in current-year sun and shade needles of unfertilized and fertilized Norway spruce [Picea abies (L.) Karst.] by simultaneous measurements of photosynthetic 02 evolution and chlorophyll a fluorescence at 20 ~ The CO2-saturated rate of 02 evolution was generally higher in sun needles of fertilized trees than in those of unfertilized trees over a wide range of incident photon flux densities (PFDs). Furthermore, the maximum photochemical efficiency of photosystem (PS) II, as indicated by the ratio of variable to maximum fluorescence (Fv/FM) was generally higher for sun needles of fertilized trees. The depression of Fv/FM during frost periods was more pronounced in sun needles than in shade needles, indicating that winter inhibition in Norway spruce is strongly lightdependent. However, the inhibition of the rate of 02 evolution at high PFDs in needles of fertilized trees during early winter was partly independent of the light regime experienced by those needles in the field, which appeared to result in a pronounced decrease in the proportion of oxidized PS II reaction centres in shade needles. A nearly identical linear relationship between the quantum yield of PS II electron transport determined by chlorophyll fluorescence and the quantum yield of 02 evolution (gross rate of 02 evolution/PFD) was obtained for the investigated types of needles during autumn and early winter. Except for shade needles of fertilized trees, this appeared to be largely achieved by adjustments in thermal energy dissipation within PS II.