Insulin-like growth factor (IGF)-I concentration in (a) serum and (b) mammary tissues of parous rats treated with insulin-like growth factor (IGF)-I (P-IGF-I), untreated parous rats (P-Un), and age-matched virgin rats (AMV). IGF-I treatment (0.66 mg/kg body weight/day) was continued for 7 days before samples were collected. IGF-I concentrations for P-IGF-I rats are combined endogenous (rat IGF-I) and exogenous (human IGF-I) values. Values are expressed as mean ± standard error. *P < 0.05: versus AMV and P-Un in panel a, and versus P-Un in panel b.