Insulin-like growth factors (IGFs) circulate in association with a family of specific binding proteins (BPs). Recently, we reported that circulating levels of IGFBP-1 and IGFBP-2 are increased in streptozotocin-diabetic adult rats and are differentially regulated in accordance with insulin and metabolic status. Since IGF BPs appear to be important modulators of IGF bioactivity in post-natal life, we asked whether serum levels of IGF BPs also might be altered in utero when the delivery of maternal nutrients is restricted and fetal growth is impaired. Bilateral uterine artery ligation or sham surgery was performed on maternal rats on d 19 of gestation (term 21.5 d). One day after ligation (d 20), fetuses were (SGA) compared to shams (3.1 +/- 1 vs 3.7 +/- 0.2 g, p less than .02) and serum levels of glucose (70 +/- 5 vs 96 +/- 6 mg/dL, p less than .01) and insulin (62 +/- 4 vs 138 +/- 14 microU/mL) also were reduced. In contrast, serum [125I]IGF-I binding activity was markedly increased in SGA litters compared to sham (65 +/- 5% maximum binding with 2.5 microL/mL SGA serum vs 14 +/- 3% for sham serum, p less than .001), and correlated with fetal weight (r = -0.539, p less than .05) and insulin (r = -0.622, p less than .05). Ligand blotting with [125I]IGF-I revealed that serum levels of IGFBP-1 (32 K) were greater in SGA than shams, while immunoblotting with specific antiserum demonstrated that levels of IGFBP-2 (34 K), the major fetal rat IGF BP, were similar in serum from SGA and shams litters. Affinity labeling and immunoprecipitation confirmed that IGF binding activity is increased in SGA, due largely to increased availability of IGFBP-1. In addition, Northern analysis of hepatic RNA revealed that the abundance of IGFBP-1 mRNA is increased in the SGA fetal rat, while hepatic mRNA for IGFBP-2 is similar in SGA and sham-operated litters. We conclude that circulating levels of IGFBP-1 and the abundance of hepatic mRNA are increased in the SGA fetal rat. IGFBP-1 may be an important modulator of IGF bioactivity and somatic growth in utero.