Spectrophotometric procedures for methemoglobin determination in eel blood were investi-gated. Two different procedures using wavelengths at 630nm and 820nm were applied to the blood samples of eel. In normal conditions, the methemoglobin level of the eel blood was esti-mated to be less than 1%by the method of Horecker and Brackett, whereas the method of Dubowski revealed much higher values because of a spectral interference between methemoglobin and cyanmethemoglobin derived. The methemoglobin level of blood sample remained unchanged at 4°C for several hours. Methemoglobin level slightly increased after 24-h exposure to nitrite at 20 ppm. At more than 30 ppm, the level remarkably increased with wide individual variations. In the recovery test, methemoglobin level returned to normal values within 24h.