The potential economic impacts of the introduction of bovine somatotropin (bST) on U.S. milk supply and demand are analyzed using a national model of Class I and Class II milk markets. The results indicate that the introduction of bST will lead to lower milk prices, higher milk production, and larger government purchases of dairy products. Unlike previous economic analyses of bST, this analysis considers both supply and demand effects of bST. The implication is that studies that ignore potential demand-side effects may produce misleading results.