Rainbow trout (Salmo gairdneri) of mean initial weight 15 g were given either a low-manganese or control diet containing 1.3 and 33 mg Mn/kg dry diet respectively. 2. Weight gains over a 24-week feeding period were the same for both groups of trout. 3. Hepatosomatic index, blood packed cell volume and haemoglobin concentration, plasma protein and the activities of aspartic aminotransferase (EC 2.6.1.1) and alanine aminotransferase (EC 2.6.1.2) were unaffected by dietary Mn intake. 4. Plasma potassium and iron levels were increased in the trout given the low-Mn diet. 5. The hepatic levels of magnesium, sodium, K, zinc, copper, Mn and phosphorus were significantly reduced in the fish given the low-Mn diet. 6. In those trout given the low-Mn diet the levels of Mn and calcium in the vertebral ash were significantly reduced. 7. The hepatic activity of Cu-Zu superoxide dismutase (EC 1.15.1.1; Cu-ZnSOD) and of Mn superoxide dismutase (EC1.15.1.1; MnSOD) in cardiac muscle and liver was reduced in the group of trout given the low-Mn diet. The fall in Cu-ZnSOD and MnSOD activities coincided with reduced tissue levels of their respective metal components.
