To test if growth hormone, well known for its strong effect on fish growth, could be incorporated into the diet, bovine growth hormone (bGH) was introduced into the lumen of the digestive tract of trout yearling (Salmo gairdneri). Radioimmunoassay (RIA) and radioreceptorassay (RRA) were used to assess plasma concentration of bGH. In our experimental conditions, the concentration of bGH peaks after 1 hour, in treated fish only, attesting to the intestinal hormone transfer (coefficient of absorption is about 0.2%). Immunocytochemistry is used to follow the hormone transfer process. Most bGH molecules absorbed by enterocytes of the distal segment are found in the vacuoles (phagolysosomes) of the apical hyaloplasm. A small part escapes the intracellular degradation and is found in the intercellular space and in the interstitial space of the lamina propria. At this level, the granule cells trapped the antigenic hormone, as did immune cells. The results of this study indicate that oral administration of GH can be potentially used in fish culture.