The transfer of growth hormone (GH) genes has opened new possibilities for the manipulation of growth in economically important fish species. However, the ectopic GH levels to optimize growth acceleration in fish, and specially in tilapia, are not known and must be determined experimentally. The tilapia GH (tiGH) cDNA was used to construct chimeric genes expressing different levels of tiGH in vitro and in vivo. These constructs were used to generate four lines of transgenic tilapia by microinjection into one-cell embryos. Different patterns and levels of ectopic expression of tiGH and IGF were detected in organs of transgenic tilapia by RNA or protein analysis. The two lines with lower ectopic tiGH mRNA levels were the only ones showing growth acceleration, suggesting that the expression of ectopic tiGH promoted growth only at low expression levels. The effect of higher ectopic tiGH levels resembled the physiologic situation of low condition factor and permitted us to postulate a model for growth acceleration in transgenic tilapia expressing ectopic tiGH.