Numerous studies demonstrated that rbST increased growth rates in several fish species, and several species exhibit GH production in tissues other than the pituitary. The role of tissue GH and IGF-I in regulating fish growth is poorly understood. Therefore an experiment was conducted to examine the effects of rbST treatment on tissue GH, IGF-I, and IGF-I receptor-A (rA) expression in rainbow trout. Rainbow trout (550±10g) received either intra-peritoneal injections of rbST (120μg/g body weight) or vehicle on days 0 and 21, and tissue samples were collected on days 0, 0.5, 1, 3, 7, and 28 (n=6/day/trt). Total RNA was isolated and assayed for steady-state levels of IGF-I, IGF-IrA, and GH mRNA using quantitative RT-PCR. Insulin-like growth factor-I mRNA levels increased in liver, gill, gonad, muscle, brain, and intestine in response to rbST treatment (P<0.10). Liver IGF-I mRNA increased (P<0.01) 0.5 day after treatment and remained elevated throughout the trial. Intestine IGF-I mRNA increased (P<0.05) in treated fish from day 1 to day 3, then decreased to day 7 and increased again at day 28, and remained elevated above control levels throughout the trial. Gill IGF-I mRNA levels increased (P<0.05) 1 day after treatment and remained elevated throughout the trial. Heart IGF-IrA mRNA levels decreased (P<0.05) while gonad GH mRNA levels increased (P<0.10) following rbST treatment. These results demonstrate that rbST treatment increased IGF-I mRNA levels in extra-hepatic tissues, and decreased heart IGF-IrA and increased gonad GH mRNA levels. Because the primary source for endocrine IGF-I is liver, the increased IGF-I mRNA reported in extra-hepatic tissues may indicate local paracrine/autocrine actions for IGF-I for local physiological functions. © 2003 Published by Elsevier Inc.