Needle yellowing is a typical symptom of declíning spruce trees [Picea abies (L.) Karst.] which grow at altitudes over 700 m a.s.l. In tiie chloroplasts of yellowing needles the thylakoidal systém was reduced. The contents of chlorophyll (Chl) a, b, carotene (a + |3) and neoxanthin in the needles decreased simultaneously, while lutein and the pigments of the xanthophyll cycle were less affected. Activities of the xanthophyli cycle were inhibited only in the advanced stages of yellowing. Yellowing processes with comparable symptoms could be induced in artificial atmospheres, simulating realistic conditions of pollution. Possible mechanisms accounting for the yellowing process were examined. Fast yellowing events occurred not only under full sunlight, but also imder red radiation (X > 600 ran). Therefore, the excited Chl probably sensitized pigment destruction. As the light-harvesting C\ň-a/b- protein complex of Photosystem 2 lost its photo- and acid-stability before yellowing occiured, chloroplasts were changed from a photostable to a photolabile state.