The present paper presents the results of a factorial experiment, Mg/K in hydroponic cultures in order to study the absorption on both elements by citrus trees. The treatments consisted of: Magnesium did not interfer with the content of K in the leaves when K was omitted or present in the nutrient solution at level 2 (704 p.p.m.). Magnesium in the nutrient solution (48 ppm) stimulated K absorption. Magnesium absorption was inhibited when K was present in the nutrient solution at levels 0 and 2. At level 1 (234 p.p.m. of K) there was an increase in Mg content in the leaves. Potassium-calcium antagonism was observed in all cases wheresas Mg-Ca antagonism was observed only the presence of higher levels of K. In the absence or presence 234 p.p.m. of K a quadratic effect of Mg was observed.