Pinfish (Lagodon rhomboides) were given different doses of copper (11) either by oral administration or by exposure to seawater containing copper.  The effects of different doses of copper on serum electrolyte concentrations, histological changes and copper concentrations in the liver and gills were examined.  Eighty pinfish with a mean weight of 100 g were divided into 2 groups of 40 fish.  The first group (group A) of 40 was subdivided into 8 groups of 5 fish / group and a subgroup was placed in seawater containing copper at one of the following concentrations in parts per million (ppm): 0; 1; 2; 3; 4; 5; 6; and 7. The second group of 40 fish (group B) was divided into 8 subgroups which were given (by stomach tube) 1 ml of seawater to which copper had been added.  A subgroup received one of the following amounts of copper in mg: 0; 5; 10; 15; 20; 25; 30; and 35. All fish were sacrificed 24 hours after dosing or exposure to copper.  Serum, liver and kidney were collected for analysis.  At the end of the 24 hour observation period fish receiving more than 20 mg of copper by stomach tube showed signs of copper poisoning (secretion of mucus, lethargy and moribundity) that became more severe as the dose increased. Fish exposed to seawater containing more than 3 ppm copper also showed signs of copper poisoning that increased in severity as the concentration of copper in the water increased. The clinical signs and changes in electrolyte values were similar for both groups A and B. It appears that copper produces the same toxic effects whether the fish ingests it or is exposed to seawater containing copper.