This chapter reviews the effects of Bovine somatotropin (BST) on dairy cow performance, milk production and composition, feed intake and efficiency, and factors affecting the animal response. BST use clearly increases milk yield and efficiency of production. This is documented in numerous single-lactation studies of the short-and long-term effects of BST on milk yields, and more recently in reports of longer studies covering as many as eight lactations. In general, increases in milk yield between the daily, 2-week, and 4- week prolonged-release forms of BST are comparable. The feeding management program may influence the long-term response to BST. Cows administered BST have a greater total nutrient requirement because of more milk produced. Energy balance and utilization in BST-treated cows, compared to controls, differ during the typical lactation cycle. BST under heat stress apparently dissipate the increased heat load through increased respiratory and skin vaporization. The nutritional-management program is a major factor influencing the response to BST.