Rates of photosynthesis at different photosynthetic photon flux densities (PPFD; 0–1530 μmol m−2s−1) were measured on three shoot age classes of one clone of Norway spruce, Picea abies (L.) Karst. exposed to three different concentrations of ozone for three seasons (1985–7) in open-top chambers. The treatments were charcoal-filtered air (CF), non-filtered air (NF) and non-filtered air + ozone (NF +). The rate of net photosynthesis was determined using an infrared gas analyzer (IRGA) connected to a small cuvette, equipped with an artificial light source. In 1- and 2-yr-old shoots the apparent quantum yield decreased with increasing shoot age and ozone concentration, while no effect was found in the current-year shoots. The decrease could probably partly be attributed to a lower efficiency of light capture due to a lower content of chlorophyll. No effect was observed on the Kok-effect or on the convexity of the light response curve in any of the treatments or age classes. The significantly higher light compensation point found in current year shoots from the NF+ treatment compared to that of shoots from the CF and NF treatments was suggested to be attributed to an increase in dark respiration. The effect of ozone was larger on photosynthesis at light saturation than on the apparent quantum yield. It is therefore suggested that ozone has a greater influence on the photosynthetic efficiency at high light conditions than at low light conditions.