Ectomycorrhizal seedlings of Scots pine (Pinus sylvestris L. cv.), inoculated with the fungus Suillus bovinus (L. ex Fr.) O. Kuntze, and non‐mycorrhizal controls were grown in growth units with a circulating culture solution. Steady‐state nutrition and constant relative growth rates were achieved by means of varied relative nutrient addition rates and free access of nutrients. Typical mycorrhizas always formed within a short period of time after inoculation. The nutrition/growth relationships were in principle similar to previous studies under steady‐state conditions: there were close linear relationships between relative addition rate, relative growth rate and internal nitrogen concentration, i.e. an equilibrium established between nutrients added and taken up. This occurred when infected and uninfected seedlings were grown separately. When grown together in the same growth unit, there are indications that the fungus influenced the exudation pattern of the uninfected seedlings. More carbon was thus provided to the unspecified microflora in the cultivation system, and it was able to grow and withhold nitrogen from the seedlings. The mycorrhizal infection did not increase the specific uptake capacity of the roots, and the fungus constituted a sink for carbon. However, the nitrogen productivity (growth rate per unit of nitrogen per unit of time) was similar for mycorrhizal and non‐mycorrhizal seedlings, so that there might be mechanisms which compensate for the carbon cost.