A greenhouse experiment of borax application to cotton cultivated on a boron deficient soil, was conducted in order to obtain data for development of future field trials. Borax was applied to pot containing 5,0kg of soil in amounts of 0, 133, 266 e 532mg. Additional treatments with higher levels of boron was applied to extra pots. The influence of boron on plant height, weight of bolls and seeds and on fiber lenght was statistically significant and of quadratic nature. The content of boron, determined either in the aboveground portion of plant or in leaf blade, increased according to the levels of fertilization used. Boron concentration in deficient plant leaves varied between 10 and 39ppm of B, with an average of 19ppm, while leaves from plants exhibiting serious symptons of toxicity showed values higher than 590ppm of B. Stunted plants, excessively branched, having enlarged and dark nodes, deformed cotyledones and leaf blades, short leaf petioles with dark rings and pith necrosis, and abnormally flowers, are related as boron-deficients. Chlorosis of leaf, in the margin or among the veins, was considered as symptom of toxicity.