This paper reports on the effect of administration of mammalian growth hormone (GH) on muscle protein synthesis as measured in white muscle using the phenylalanine flooding technique. The effect of exogenous GH was compared with that of insulin and prolactin, and with endogenous GH. The rate of protein synthesis in white muscle of rainbow trout 6 h after the injection of bovine GH or bovine insulin was twice (2.6 and 2.9% d(-1)) that of the control saline-injected fish (1.2% d(-1)). A metabolic effect of GH, as observed with insulin, is suspected. The rates of change in body weight and body length and the fractional rate of protein synthesis in muscle of rainbow trout were enhanced by mammalian GH administration. The effect of GH on muscle RNA/protein ratios was not significant. An opposite effect of antibodies against salmon GH (Lebail et al. 1989) on growth rate and muscle protein synthesis rate was found in rainbow trout. It is suggested that the effects of exogenous and endogenous GH on capacity and efficiency of muscle protein synthesis were similar. The long-term effects of mammalian GH on presmolt Atlantic salmon was also tested. The same trends were found with ovine prolactin supplementation in Atlantic salmon but not as high as those observed with ovine GH.