Objective To study temporal expressions of insulin-like growth factor-Ⅰ (IGF-Ⅰ) and insulin-like growth factor-1 receptor (IGF-1R) in the developing brain of rat and explore their functions in nervous system development.Method IGF-Ⅰ and IGFR-1 rnRNA expressions in the developing brain of rat (from E10.5-E18.5) were studied by Real-time PCR.Results (1) The relative quantities (RQ) of IGF-Ⅰ mRNA frorn E10.5 to E18.5 were 1, 2.6, 13.4, 15.5, 9.2, 51.5, 4.6, 3.5 successively.There were two peaks of IGF-Ⅰ mRNA expression: the first one was on E14.5 when the RQ of IGF-Ⅰ mRNA was 15 times more than on El0.5, the second one was on El6.5 when the RQ of IGFR-1 mRNA was 51 times more than on El0.5.(2) The relative quantities of IGF-1R mRNA from E1 0.5 to E1 8.5 were 1, 1, 1.5, 2.2, 1.5, 1.5, 1.8, 0.24, 0.24, 1.3 successively.The expression level remained relatively constant and there was no significant expression peak of IGFR-1 mRNA.Conclusion IGF-Ⅰ maybe have different important roles in regulating neural stem cell proliferation and differentiation according to different expression levels during rat brain development.