An attempt was made to observe the changes in the oxidation-reduction potential of semen under anaerobic condition, and to demonstrate the relationship between the potential and the characteristics of semen. Electrode potential measurements were obtained in diluted semen and in various diluents by employing a platinum electrode enclosed within a slightly modified Michaelis vessel. This was closed by a glass stopper which contained an electrode, an inlet and outlet tubes for nitrogen gas, and a saturated agar KCl bridge connected with the calomel half cell. Number of spermatozoa was adjusted to make 5×10^8 per milliliter and five milliliters of test sample were used for studying the electrode potential measurements. After the introduction of test sample into the vessel, 0.025 milliliters of 0.5 mM methylene blue were added to the sample. Most of the air was then replaced with purified nitrogen gas, the vessel stoppered and calomel half cell were set up in an incubating bath kept at 35℃. All potential measurements were made on a Hitachi-Horiba, F-5 pH meter. The electrode potential of diluted semen decreased rapidly during approximately first 30 minutes, accompanied by a slow down for additional 30 minutes, after which the potential showed a constant tendency for Eh in general. When the diluted semen was aerated for several seconds by means on babbling air through a outlet tube for nitrogen gas after the potential had reached steady level, it caused a sudden rise in electrode potential. However, this rise rapidly fell to about the original, level within 20 minutes following introduction of nitrogen gas. The treatment of undiluted semen at 60℃ for 30 minutes resulted in marked decrease in the reducing ability of semen as compared to a control. The electrode potentials of 0.9 % sodium chloride, Ringer's solution, isotonic phosphate buffer, Lake's solution, isotonic phosphate buffer containing antibiotics, and hypertonic phosphate buffer （1.5 times isotonic concentrations） containing antibiotics were measured. Eh values for these diluents were 159, 153, 129, 111, 47, and 45 mv, respectively. The addition of 0.5 mM neutral red to a Ringer's solution in order to lower the Eh of the solution resulted in improved motility and fertility of semen stored at 0℃ for 5 hours.