The effects of standard copper sulphate (0.5 ppm) and formalin (200 ppm) bath treatments on haematocrit values, total haemoglobin, blood glucose and the activity of lactate dehydrogenase (LDH), hydroxybutyric dehydrogenase (HBDH), glutamate pyruvate transaminase (GPT) and glutamate oxaloacetate transaminase (GOT) in the plasma from apparently healthy rainbow trout are described. A single copper sulphate treatment produced statistically significant increases in mean values of all parameters measured. GPT activity showed the greatest increase, two-fold, and was still elevated after 24 h. Formalin only significantly affected haematocrit (Hct) and GOT activity. The results of clinical tests are correlated with a study of short term changes in liver histopathology. Recovery after a single treatment and degenerative changes resulting from repeating treatments are described. Implications for defining treatment regimes to minimize chemotherapeutic stress are considered.