Seedlings of Norway spruce (Picea abies(L.)) were grown at 335 and 1000 μl CO21−1for 118 days in growth rooms at different irradiance levels. Photon flux density ranging from 8.6 to 34.6 mol m−2day−t(PAR) was given either as constant light or as alternating levels in intervals of two or six hours. CO2enrichment increased the plant dry weight from 36% to 105% by increasing photon flux density from 8.6 to 25.9 mol m−2day−1. At constant light the dry weight apparently reached its maximum at a photon flux density of 25.9 mol m−2day−t. At the lower radiation levels alternating in CO2enriched air gave slightly higher dry weights compared to constant light levels. At the highest radiations the effect on dry weight was the opposite. High CO2concentration and 300 μmol m−2s−1constant light (25.9 mol m−2day−1) gave the best growth and quality of plants. Top, root, stem and foliage weight were proportionally affected. Shoot length was enhanced by CO2enrichment. Shoot weight per cm was substantially increased both by CO2enrichment and increasing photon flux density.