The effects of dam parity, age at weaning, and preweaning diet were examined in the ontogeny of serum insulin-like growth factor-I (IGF-I) concentrations in foals. Foals born to 13 primiparous and 19 multiparous draft-cross mares were weighed and bled near birth. About one-half of the foals in each group were weaned early (about 13 wk old); the remaining foals were weaned late (about 16 wk of age). Pooled values for serum IGF-I concentrations between birth and 17 wk of age were higher (P < 0.065) for foals born to multiparous (386 ng/ml) than to primiparous mares (237.5 ng/ml). Colts (378 ng/ml) had higher (P < 0.05) serum IGF-I concentrations than fillies (254.5 ng/ml), regardless of dam parity. Colts (173.5 kg) also tended (P = 0.12) to be heavier than fillies (159.2 kg). Weaning, whether at 13 or 16 wk of age, reduced (P < 0.05) growth rates and serum IGF-I concentrations. Serum IGF-I values recovered to preweaning values within 1-3 wk postweaning concurrent to an improved weight gain. Fifteen 1-d-old foals in a second study were fed milk replacer for 7 wk and were compared with five foals that nursed their mares for 8 wk. During the first 2 wk, replacer-fed foals (0.46 kg/d) did not gain as rapidly (P < 0.03) as mare-nursed foals (1.73 kg/d). The associated serum IGF-I values for replacer foals (139.4 ng/ml) were lower (P < 0.0001) than values for mare-nursed foals (317.4 ng/ml). Despite similarity in gains for both groups there-after, serum IGF-I concentrations of replacer-fed foals were only 36 and 60% of values obtained for mare-nursed foals at 8 (weaning) and 18 wk of age, respectively. The intrinsic differences between mare-nursed and milk-replacer foals in serum IGF-I concentrations persisted to 1 yr of age despite similarities in dietary management and body weight of the foals. At 1 yr of age, the serum IGF-I concentration of mare-nursed foals (1,203 ng/ml) was 48% higher than that of replacer-fed foals (815 ng/ml). These data indicate that dam parity, sex of foal, and preweaning nutrition affect the ontogeny of serum IGF-I concentration in the foal. The chronic, persistent difference in serum IGF-I values created by the early nutritional management of growing animals has implications in the interpretation of longitudinal serum IGF-I studies in all species.