ABSTRACT. We measured the blood plasma testosterone (T) levels and superoxide dismutase (SOD) and catalase activities in the seminal plasma of the ejaculates of 5 normal (2-5 years old) and 5 asthenozoospermic (AZ-) (3-5 years old) Beagles. Sperm ejaculated by AZdogs was incubated for 3 hr in Eagle's MEM only (controls) or Eagle's MEM containing 100 units/ml of SOD or catalase. Sperm motility was examined during incubation. The mean (± SE) plasma T level of the AZ-dogs (1.2 ± 0.2 ng/ml) was significantly lower than in the normal dogs (2.5 ± 0.2 ng/ml) (P<0.005). The mean (± SE) seminal plasma SOD and catalase activities (18.8 ± 1.9 and 0.5 ± 0.1 unit/g protein, respectively) were significantly lower in the AZ-dogs than in the normal dog (43.3 ± 2.5 and 2.2 ± 0.4 unit/g protein, respectively) (P<0.001 and 0.01, respectively). The motility of sperm incubated in Eagle's MEM containing SOD or catalase was significantly higher than that of control sperm incubated in only Eagle's MEM after 2 or 3 hr of incubation (P<0.05). The results of this study indicate that poor T secretion by the testes and low antioxidant enzyme activities are related to AZ in the dog. KEY WORDS: canine, catalase, seminal plasma, SOD.J. Vet. Med. Sci. 69(2): 133-136, 2007 Seminal plasma is known to contain reactive oxygen species (ROS) produced by testicular tissue [18] and sperm [2,20], and elevated seminal plasma ROS concentrations in both humans [1,16] and dogs [22] have been reported to be a cause of oligozoospermia, asthenozoospermia, and teratozoospermia. Low sperm motility and morphologically abnormal sperm occur as a result of sperm plasma membrane dysfunction caused by ROS [7].Superoxide dismutase (SOD) [2,6,9] and catalase [6,19,24] are the main antioxidant enzymes in seminal plasma that prevent increases in ROS concentration in seminal plasma and protect the sperm against damage and oxidative stress caused by ROS. The SOD [6,8,18] and catalase [17,24] in seminal plasma are produced by the testis, epididymis, accessory reproductive organs, and sperm, and they are able to maintain sperm motility for a long time [6,8]. Seminal plasma SOD has been reported to have the same effect on canine sperm [5]. However, the cause of spermatogenic arrest in the dog is unknown [12]. In the present study, we examined the interaction between peripheral blood plasma testosterone (T) levels and the activities of seminal plasma SOD and catalase in the dog. Based on the results of this study, we suggested some causes for spermatogenic dysfunction in the dog. MATERIALS AND METHODS Animals:Ten male Beagles aged 2-5 years were used in this study. They were cared for in our university and housed in pens with ample runs. Commercial dry dog food was provided twice a day, and the dogs were given free access to water. All animals were maintained according to the guidelines of the Animal Care and Use Committee of the Nippon Veterinary and Life Science University.The semen quality of the dogs was examined 3 times at one-week intervals. Five of the dogs (3-5 years...