It is quite comon in brazilian rubber plantations the occorence of boron deficiency symptoms on the leaves associated with N, P, K, and Mg fertilizer programs. In a greenhouse experiment the daily aplication of five levels of boron (0.5; 1.0; 1.5; 2.0 and 2.5 ppm) to the substract induced toxicity symptoms to the leaves of the young rubber trees. Tips and marginal necrosis of the leaves occured when the boron concentration in the subtract was between 1.0 and 2.5 ppm of boron. Recently in an experiment carried out in order to obtain the macro and Zn, Mn deficiencies on grafted plants with clone RRIM 600, the boron was withdraw from the nutrient solutions in order prevent toxic effects in the plants. After six months symptoms appeared which consisted of a halt on growth; the terminal buds die and an exudation of latex started. After the supply of 0.1 ppm of boron to the nutrient solutions the plants began to growth again and numerous branches started to appear in the plants.