To study developmental regulation of somatostatin(SS) in rat mammary gland, this thesis selected virgin, days 6,12,18 of pregnancy, days 6,12,18 of lactation Sprague-Dawley rats as objection. We had studied the localization and expression of SS in mammary gland, the changes law and the corelation SS to others factors at all stages of rat mammary development, also further demonstrated the regulation roles of SS on mammary development using SS and its exhaust agents (cysteamine,CS) by the immunohisto- chemical(IHC), the radicalization immunity assay(RIA), the enzyme link immunity absorption assay(ELISA) and the RT-PCR methods. The results sees below:1. The localization and expression of SS in the mammary glands (MG) of the rat by IHC and RT-PCR methods. The results showed that intense staining for SS throughout the cytoplasm of the epithelial cells and high power view of the secretory alveoli in mammary gland of virgin, days 6,12,18 of pregnancy, days 6,12,18 of lactation Sprague-Dawley rats and there exists the SS mRNA expression at all stages of rat mammary development.2. We had studied the correlation of SS to mammary development by RIA and ELISA methods.The results showed that the content of SS decreases along with mammary development. The results by RIA showed that the content of SS is highest at virgin, its content at pregnancy and lactation decreases little by liettle.The results by ELISA showed that the SS content at days 6 of pregnancy is higher than those at virgin(P>0.05).Compare with each group at lactation, difference significance(P<0.05).The SS content at days 12 of lactation increase little, but compare with each group at lactation, difference insignificance(P>0.05).3. The correlation SS to some relative factors during mammary development by RIA and RT-PCR. We found :3.1 The results showed that estrogen level in serum is higher in virgin than that in pregnancy and lactating, the estrogen level in mammary gland is 12.36 pg/mg tissue protein in virgin, decreases in pregnancy, increases after parturition, decreases in lactating. We found the estrogen receptor mRNA expression in virgin, pregnancy and lactating, estrogen receptor mRNA expression level decreases along with mammary development. These results suggested that SS can regulate estrogen receptor mRNA expression in two direction, Up-regulate at pregnancy and down-regulate at lactation. at same time can participate in mammary epithial development and lactation by up and down regulation estrogen level in serum and mammary