A 3:30 Ca:Mg ratio in the nutrient solution produces in tomato seedlings symptoms of Ca<sup>2+</sup> deficit owing to excessive accumulation of Mg<sup>2+</sup> ions and the depressed Ca<sup>2+</sup> accumulation. As a result of this a decrease in dry weight increment and protein content is observed together with inhibition of nitrate reductase activity. A doubled Ca<sup>2+</sup> dose in the nutrient solution, that is a change in the Ca:Mg ratio to 6:30 abolishes the external symptoms of Ca<sup>2+</sup> deficit and reduces Mg<sup>2+</sup> accumulation, that of Ca<sup>2+</sup> ions remaining unchanged. At the same time an enhanced activity of nitrate reductase appears, reaching values close to those in control plants. Tomato seedlings grown on a 3-fold increased Ca<sup>2+</sup> dose (Ca:Mg = 9:30) did not differ at all from the control ones. An in-crease in calcium concentration in the nutrient solution, the high magnesium dose remaining unchanged, causes enhanced K<sup>+</sup> accumulation, and this may affect nitrate absorption and reduction. It would seem that Ca<sup>2+</sup> deficit in plant tissues induced by excessive Mg<sup>2+</sup> accumulation with unsuitable Ca:Mg ratio in the nutrient solution in cause of disorders in NO<sub>3</sub> nitrogen assimilation.