Objective To investigate the effect of insuline-like growth factor-I (IGF-I) on proges- terone genesis and regulation. Methods Cytotrophoblast cells were collected by trypsin-collagenase digestion and percoll gradient centrifugation for primary culture. After stimulated with different concentrations(100 μg/ml,10 μg/ml,1 μg/ml,0.1 μg/ml) of IGF-I at the same time and with different duration(12 h,24 h,48 h,72 h) of IGF-I with the same concentration, progesterone levels in the media were measured by radioimmunoassay. Simultaneously, semiquantitative reverse transcriptase polymerase chain reaction(RT-PCR) was ap- plied to determine the expression of low density lipoprotein receptor (LDLR) mRNA. Results Progesterone levels correlated positively with IGF-I along with the IGF-I concentration increasing, progesterone level began to increase at 12 h, and reached the climax at 48 h when cultured with 100 ìg/L IGF-I. The expression of LDLR mRNA was detectable in every group and accordant with variation of progesterone level. Conclusion Progesterone secretion has time- and dose-dependent effect on IGF-I, and IGF-I can up-regulate the expression of LDLR mRNA. IGF-I may play an impor- tant role in promoting secretion of progesterone in trophoblast cells. Objective To investigate the effect of insuline-like growth factor-I (IGF-I) on progesterone genesis and regulation. Methods Cytotrophoblast cells were collected by trypsin-collagenase digestion and percoll gradient centrifugation for primary culture. After stimulated with different concentrations ( 100 μg / ml, 10 μg / ml, 1 μg / ml, 0.1 μg / ml) of IGF-I at the same time and with different duration (12 h, 24 h, 48 h, 72 h) the same concentration, progesterone levels in the media were measured by radioimmunoassay. Simultaneously, semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) was ap- plied to determine the expression of low density lipoprotein receptor (LDLR) mRNA. with IGF-I along with the IGF-I concentration increasing, progesterone level began to increase at 12 h, and reached the climax at 48 h when cultured with 100 μg / L IGF-I. The expression of LDLR mRNA was detectable in every group and accordant wit h variation of progesterone level. Conclusion Progesterone secretion has time- and dose-dependent effect on IGF-I, and IGF-I can up-regulate the expression of LDLR mRNA. IGF-I may play an impotence role in promoting secretion of progesterone in trophoblast cells.