Effects of exogenous pST and bST on metabolic and growth factor variables were examined in three studies with lighthorse mares (455 to 545 kg). In Study 1, eight mares received five s.c. injections of bST or pST (30 mg/d). In Studies 2 and 3, five mares received one s.c. injection of a prolonged release formulation designed to deliver 500 mg of bST (Study 2) or pST (Study 3) over 14 d. Blood samples were collected for several days before injection to establish baseline values, at frequent intervals during treatment, and for several days thereafter. In all studies, blood urea nitrogen concentrations were decreased (P < .001) and insulin-like growth factor I (IGF-I) concentrations were increased (P < .001) within 48 h after bST or pST injection relative to pretreatment values. Similarly, insulin and glucose were increased (P < .001) relative to pretreatment values, after bST or pST administration. In Studies 2 and 3, circulating ST concentrations were increased (P < .001) for at least 14 d after injection, despite severe local tissue reactions at the prolonged release formulation injection site. Insulin-like growth factor I ligand blotting of serum revealed bands with molecular weights (MW) of 45, 32, 30, and 18 kDa, and two bands of > 96 kDa. These results indicate that 1) bST and pST are biologically active in horses, which respond metabolically to exogenous ST in a manner similar to other mammalian species, 2) circulating IGF binding proteins are present in horses, and 3) the commercially available dairy cow product POSILAC (Monsanto, St. Louis, MO) is not appropriate for the delivery of bST in horses due to injection site reactions accompanying the administration of the oil-based prolonged release formulation.