Yellow eels (Anguilla anguilla L.) with an average weight of 60 g were used in this experiment. The fish were caught in June/July at the Aveiro Lagoon on the Portuguese West Coast, transported to the Department of Biology, Aveiro University, and kept in aerated aquaria for 1 week before the experiment started. The eels were then exposed for 1 and 3 weeks to 75 and 50% of the kraft pulp mill effluent. The eels exposed to the kraft pulp mill effluent developed an increase in red blood cell number per cubic millimeter and several biochemical changes, such as an increase in plasma lactate and sodium and a decrease in plasma pyruvate and potassium. Histological examination of the experimental eels exposed to the 50% kraft pulp mill effluent revealed deep alteration of the tissue structure, such as disruption of the skin and edematous hypertrophy of covering epithelial cells in secondary gill lamellae. The kidney had damage of the renal tubules. The liver developed necrosis supported by a significant decrease in GOT and GPT activity. The spleen had an increase in blood content as well as in pigment centers. Previous results indicated the kraft pulp mill effluent causes tissue damage and consequent metabolic changes in the eel Anguilla anguilla L.