Recombinant bovine growth hormone (rbGH) increases growth rates in rainbow trout, Oncorhynchus mykiss, and this response is thought to be under the control of the GH-IGF axis, as it is in mammals. However, the mechanisms regulating fish muscle growth are poorly understood. Therefore, an experiment was conducted to examine the effects of rbGH on growth-related hormones in rainbow trout. Rainbow trout (550±10 g) received an intraperitoneal injection of rbGH (120 μg g-1 BW) or vehicle on days 0 and 21. Blood samples were collected on days 0, 0.5, 1, 3, 7, and 28 and assayed for rainbow trout growth hormone (tGH), rbGH, and IGF-I. As expected, rbGH levels increased (P<0.05) in circulation 12 h after treatment and continued to increase (P<0.05) from day 0 to day 7 after treatment. Unexpectedly, levels of rbGH detected at day 28, 7 days after the second injection, were lower than those at day 7. Corresponding to this observation, anti-rbGH antibodies were detectable in serum from treated fish at day 28, but not at day 7. We suggest that the low levels of rbGH detected at day 28 were due to increased clearance of rbGH caused by the anti-rbGH antibodies. Treated fish also exhibited increased serum IGF-I levels (P<0.01) following rbGH injection, while endogenous tGH did not change (P=0.28). These results suggest that the endogenous negative feedback control loop described in mammals is not activated by rbGH in rainbow trout, as tGH was unaffected by increased circulating rbGH and IGF-I. However, consistent with previous reports, rbGH does increase circulating IGF-I over time and rbGH is detectable throughout the 3-week injection period. To our knowledge, this is the first report demonstrating specific antibody production following an exogenous rbGH injection in fish. © 2005 Elsevier B.V. All rights reserved.