- Abstract Two experiments were carried out to study fish growth and several other metabolic parameters under three main effects: dietary protein level (40% vs. 21%); water temperature (26 vs. 17°C), and intraperitoneal administration of carp growth hormone (cGH) (5 μg/g body weight/week vs. saline). Whereas both higher protein levels and elevated temperature promoted growth, the effect of cGH administration was highly dependent on protein level, the largest growth-promoting effect being found with low-protein diets. The dependence was particularly significant in the first experiment, in which the amount of feed provided was highest (5% vs. 2% of body weight/day). At the lower water temperature, fish in all treatments showed a significantly elevated hepatosomatic index. Protein level and cGH administration effects were unclear. The protein: fat ratio in the body contents was decreased by the high-protein diet and lower water temperature, whereas cGH had no effect. In contrast, the glycogen: fat ratio depended mainly on temperature—yielding higher ratios at the lower temperature, and not on cGH or protein levels. Carp GH administration decreased AcCoA carboxylase activity in the liver. Water temperature did not change the level of GH or T 3 in the plasma. Protein level did not influence GH but increased T 3 , whereas GH expression in the pituitary gland was downregulated by the administration of cGH and by the low-protein diet.