To estimate possible changes in the blood of Japanese eel attributable to sex and maturity differences, hematocrit, hemoglobin content, erythrocyte count and plasma protein were examined quantitatively and electrophoretically for used eel samples with or without hormone injection. At present, maturity of the eel samples treated with hormone was divided into three stages, immature, maturing and mature, by ratios of the gonad and body weights. In male treated with gonadotropin there was no significant change in erythrocyte count and plasma protein from the immature stage to the mature, but gross changes occurred in hematocrit and hemoglobin content at the mature stage. In female treated with gonadotropin, estrogen or releasing hormone, hematocrit, hemoglobin content and erythrocyte count were reduced greatly with gonad maturation. Sexual maturation of the female eel with gonadotropin and estrogen seemed to be accompanied by profound changes in plasma protein, average values of which were 18.4, 16.7 and 7.9 in g/dl at the immature, maturing and mature stages, respectively. In female, starch gel electrophoretic pattern of plasma protein changed to a great extent with maturation.