Technical Abstract: Numerous studies have established that recombinant bovine Somatotropin (rbST, aka bovine growth hormone) stimulates growth in the rainbow trout. However, the effects of rbST on target tissue gene expression are not well characterized. In the current study, we used Posilac® (Monsanto Co., St. Louis, MO) to assess the effects of rbST treatment on liver and muscle transcript levels in fish. Transcript abundance was measured in the current study using the GRASP 16 K cDNA microarray. Rainbow trout were selected from four families, two characterized as high growth rate and two as low growth rate. A total of 180 transcripts were identified as differentially expressed across tissues and growth rate. The largest proportion of the transcripts represented novel transcripts; immune and metabolism related genes represented the next largest contribution. The immune related genes were primarily modulated in the liver and indicate activation of a non-specific immune response in the rainbow trout following GH treatment. The metabolic genes include lipid metabolism, oxidative phosphorylation and one carbon metabolism pathway transcripts. A selection of the genes identified with the microarray and insulin-like growth factors, growth hormone receptors and myostatin transcripts were measured in the brain, kidney, intestine, stomach, gill and heart following GH administration. In general, transcripts identified as differentially regulated in the muscle on the microarray showed similar direction of expression in other extra-hepatic tissues. Our results indicate that short term GH treatment activates the immune system, shifts the metabolic impetus, and modulates growth regulating genes.