Objective Toinvestigatetheeffectofchronicrecombinantgrowthhormone(rhGH)stimulationonhepaticSTAT5in healthy young mouse and its molecular mechanism .Methods 32 normal young mice were divided into 4 groups :PBS ,PBS+single rhGH ,chronic rhGH ,chronic rhGH+ single rhGH groups .The mice in the treatment groups(chronic rhGH and chronic rhGH +single rhGH )received intraperitoneal injection of rhGH at 1 μg/g body weight once daily for three weeks ;the mice in the control groups(PBS and PBS+single rhGH) were injected with sterile phosphate buffered saline(PBS ,0 .1 mol/L) .At next day 8:00 after 16 h of injection of last rhGH and PBS ,at 30 min before killing mouse ,PBS 0 .1 mol/L was injected in the PBS group and the chro-nic rhGH group ,while the single dose of rhGH 1 μg/g was injected in the PBS+single rhGH and the chronic rhGH+single rhGH group .Results Compared with the control mice(PBS group and PBS+single rhGH group) ,the body mass in the rhGH stimulation mice (chronic rhGH group and the chronic rhGH + single rhGH group) was significantly increased ;compared with the PBS group ,the base p-STAT5 level within the hepatic GH signal pathway in the chronic rhGH was significantly decreased ;the combi-ning capacity of hepatic STAT5 and DNA detected by EMSA in the chronic rhGH group was significantly lower than that in the PBS group ;in the mice injected by single dose of rhGH 1 μg/g at 30 min before killing mouse ,the hepatic p-STAT5 level in the chronic rhGH+single rhGH was significantly decreased compared with the chronic rhGH + single rhGH ;compared with the con-trol group ,the CIS content in the chronic rhGH was obviously increased ,while the SOCS-2 content had no obvious difference .Con-clusion Chronic rhGH stimulation can inhibit the phosphorylation of hepatic STAT 5 in normal young mouse ,which mechanism could be related with the increase of CIS content .