We have studied the correlation between nitrate and potassium uptake in spring and autumn in two deciduous tree species: walnut and sycamore maple. Two-year-old trees were transplanted in early spring and cultivated on hydroponic recirculating nutrient solution systems in a greenhouse. Uptake of nitrate and potassium was surveyed daily during three consecutive weeks in mid-spring (i.e. during vegetative growth) and mid-autumn (i.e. during bud dormancy). Nitrate and potassium uptake was expressed per unit tree leaf area to account for tree size effect. Our results show that nitrate was much more absorbed than potassium in both species and its uptake remained nearly the same in spring and autumn. Contrary to this, potassium uptake was strongly reduced in autumn as a consequence of a strong reduction in vegetative growth. Although potassium uptake was strongly affected by seasonal variation in vegetative growth demand in both species, a positive and strong correlation between nitrate and potassium uptake was maintained in both species whatever the season. Essentially, any reduction in nitrate uptake as a consequence of decrease in nitrate availability in nutrient solution induced concomitantly a reduction in potassium uptake even if its concentration in the nutrient solution was sufficient to ensure potassium uptake. The results are discussed in the light of the accompanying role of potassium for nitrate uptake in plants. A conceptual scheme for internal/external potassium cycling in plant is proposed to explain the seasonal variations in nitrate and potassium uptake correlation in both deciduous tree species.