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Willows (Salix x dasyclados) were grown for 4 months in growth chambers at four nutrient and CO levels, and photosynthesis measurements were made during the latter half of this period. Photosynthesis became saturated at lower light intensities at low CO concentrations than at higher ones. The effect of CO concentration on photosynthesis was greater at higher temperatures. The willows grown at the highest CO concentration (1000 ppm) had a lower photosynthetic capacity than the others when measured at various concentrations. The effect of nutrient status on photosynthesis clearly increased with rising CO concentrations. Although photosynthetic acclimation took place to a certain extent at higher CO concentrations, photosynthesis still remained higher the higher the growth concentration was. At each CO level photosynthesis increased contemporaneously with leaf nitrogen content, but at each fertilization level a rise in CO concentration slightly increased photosynthesis and reduced the nitrogen content. The relative increase in photosynthesis achieved by a rise in CO was greater than the corresponding increase in biomass growth, whereas the effect of fertilization was greater on biomass growth than on the rate of photosynthesis in the same willows.