Abstract: IGF-I mRNA expression of 7-month old juvenile and 2-year old adult common carp were determined by RNase Protection Assay. Results showed that the hepatic IGF-I mRNA level of adult fish was significantly higher than that of 7-month old juvenile fish; while in other tissues there was no significant difference between the two fish groups except that kidney IGF-I mRNA level of adult fish was higher than that of juvenile fish. After twice i. p. injections of LHRH-A, the levels of serum GH and hepatic IGF-I mRNA in fish of both ages increased greatly, and the IGF-I mRNA level of gonadal tissue in adult fish increased when compared with controls. No changes in amount of IGF-I mRNA in other tissues were observed after injections of LHRH-A. It is suggested that the level of hepatic IGF-I mRNA expression changes with different developmental stages; LHRH-A may stimulate the production of IGF-I by promoting the synthesis and release of pituitary GH; and probably, LHRH-A stimulates the IGF-I expression in gonads by other mechanism.