The discussion of effects of recombinant bovine somatotropin (rbST) administration on the metabolism of lactating dairy cows presented is divided into two parts: short term and long term. Short-term effects are evident during the initial period of treatment when milk production is increased but feed intake has not yet increased. During this period body reserves are mobilized to support the higher rates of milk production. This phase is very similar to the early lactation condition, when energy requirements for milk production also exceed energy intake. Two notable exceptions to this analogy are that glucose and beta-hydroxybutyrate concentrations are not altered during rbST treatment but are influenced by stage of lactation. After the initial phase of rbST treatment, feed intake increases to compensate for the increase in milk production. During this phase the metabolism of cows treated with rbST essentially mirrors the metabolism of untreated cows producing comparable amounts of milk.