It is quite common in brazilian rubber plantations the occurence of boron deficiency symptoms on the leaves associated with N, P, K, Mg fertilizer programs. A simple experiment involving the daily aplication of five levels of boron (0.5, 1.0, 1.5, 2.0, 2.5 ppm) to established rubber seedlings was carried out in a greenhouse with washed sand to determine the effects of excess boron associated with toxicity symptoms. The symptoms were observed nine days after commecement of treatments. The concentration of boron in the leaves were from 316 to 1300 ppm. In the stem and roots the concentrations of boron were lower from 21 to 81 ppm of boron on dry matter. Tip and marginal necrosis of the leaf occured when the boron concentration in the substract were 1.0 to 2.5 ppm of boron. The rubber plants exhibet early defoliation. The authors concluded that a very carefull atention should be taken when the application of boron under field condition in rubber plantations .