Preterm, human, placental fibroblasts exhibit growth rates, in vitro, that vary with gestational age. The observed increase in proliferation rate is associated with enhanced mitogenic responsiveness to IGF-I. IGFBPs can either potentiate or inhibit IGF action at the cellular level. The production of IGFBPs by placental fibroblasts was studied as potential modulators of their responsiveness to IGFs. Human placental fibroblasts were obtained at various gestational ages and maintained in culture. IGFBP-3 protein and mRNA expression were assessed by Northern and ligand blot analyses. First, media conditioned by fibroblasts, in culture, were subjected to ligand blot analysis. Multiple species of IGFBPs were present in each cell line tested. IGFBP-3, migrating as a doublet at approx. 38/42 kDa, was the predominant IGFBP species present. Other IGFBPs of 22-35 kDa were also present. The secretion of IGFBP-3 exhibited a marked decrease at 10-15 weeks gestation relative to 8-9 week fibroblasts but began to increase again by 19 weeks. We next studied the expression of IGFBP-3 mRNA. Total cellular RNA was obtained from rapidly growing cells and subjected to Northern analysis. Placental fibroblasts exhibited decreased steady state levels of IGFBP-3 mRNA at 10-15 weeks gestation consistent with its decreased protein expression. The ability of IGFBP-3 to influence IGF-1 stimulated DNA synthesis was studied in 10 week placental fibroblasts as measured by [3H]thymidine incorporation. IGFBP-3 inhibited IGF-1 (3.3 nM) stimulated DNA synthesis in a dose-dependent manner when added simultaneously with IGF-1 or preincubated with the cells for 48 h prior to the addition of IGF-1. By contrast, maximum effective concentrations of IGFBP-3 (52 nM) potentiated the effect of IGF-1 50-200% when preincubated with bovine fibroblasts for 48 h prior to the addition of IGF-1. These data suggest that IGFBP-3 production is developmentally regulated in human placental fibroblasts and inhibits their mitogenic response to IGF-1. The regulated expression of IGFBP-3 may contribute to the altered growth rate and IGF responsiveness exhibited by placental fibroblasts, in vitro.