Insulin-like growth factors (IGF) and their specific binding proteins (IGFBPs) are believed to be important regulators of fetal growth. IGFBP protease which proteolyzes IGFBPs and changes the biochemical properties of IGFBPs is also presumed to be involved in fetal growth. The aim of this study is to elucidate the physiological significance of IGFBP protease in fetal growth and regulators of protease in placenta and decidua. The intact IGFBP-3 was proteolyzed into fragments when pregnant serum was incubated with 125I-IGFBP-3. IGFBP-3 protease activity showed a marked increase at 5 weeks of gestation and reached a plateau in maternal circulation at 15 weeks of gestation. These changes in protease activity correlated with the profiles of IGFBPs in the maternal circulation analyzed by Western ligand blot, where the IGFBP-1 is only the dominant IGFBP. The intact IGFBP-3 was proteolyzed when culture media of decidual cells were incubated with 125I-IGFBP-3, but was not proteolyzed when culture media of trophoblast cells were incubated with 125I-IGFBP-3. Decidual protease activity was slightly increased by IGF-I and completely inhibited by progesterone. The protease activity was more increased in the mothers with growth retarded infant than in those in the mothers with normal growth infants, suggesting that the protease activity is elevated in compensation for the impaired fetal growth. These results suggest that increased protease activity in maternal blood may be involved in the fetal growth indirectly by reducing the binding activity of IGFBP-3 to IGF-I, and that protease activity in maternal blood may be derived from decidua that is regulated by placental hormones.