ABSTRACT. Two azoospermic dogs with high plasma estradiol-17β (E 2 ) levels were subcutaneously injected with an aromatase inhibitor (AI), 4-androstene-4-ol-3,17-dione, 2 mg every other day for 4 weeks. Before the AI treatment the plasma E 2 levels of the two dogs (21 and 22 pg/ml, respectively) were higher than those of 2 normal dogs (8.1 and 12.3 pg/ml), and they fell to 11-17 pg/ml between 1 and 4 weeks after the start of AI treatment. The plasma testosterone levels after the start of AI treatment had increased to 2.1-3.1 ng/ ml. A small number of sperm were detected in the semen of the two dogs between 3 and 6 weeks after the start of AI treatment. Th ese results indicate that the testicular function of infertile dogs with high plasma E 2 levels can be temporarily improved by AI therapy. KEY WORDS: aromatase inhibitor, azoospermia, canine.J. Vet. Med. Sci. 65(12): 1343-1345, 2003 A portion of the testosterone (T) produced by the testis is converted to estradiol-17β (E 2 ) by aromatase enzyme activity [11]. In the testes of many species [2,17], including dogs [3], the E 2 -secreting cells are the Sertoli cells and/or the Leydig cells [13,20]. In men [6,19] abnormally increased testicular E 2 production causes spermatogenic dysfunction, and long-term E 2 administration has been shown to inhibit spermatogenesis in dogs [8,15]. Aromatase inhibitor (AI) blocks the aromatization of androgen to E 2 by inhibiting aromatase enzyme activity [1,18], and AI has been reported to be effective in treating spermatogenic dysfunction in men with high plasma E 2 levels [4]. In the present study, two azoospermic dogs with abnormally increased plasma E 2 levels were treated with AI to improve poor semen quality. Plasma E 2 levels after administration of follicle stimulating hormone extracted from porcine pituitary gland (FSH-P) were measured to investigate the main E 2 -secretory cells in canine testes.The two azoospermic dogs used to assess the effects of AI treatment were a Miniature Poodle (Dog 1) and a Beagle (Dog 2), aged 5 and 2 years, respectively. Dog 1 was owned by a breeder, and Dog 2 was cared for in our university. None of a few bitches mated with either of the two male dogs had conceived. Four semen specimens were collected by digital manipulation with a teaser bitch at one-week intervals. Each specimen was examine for total semen volume, total number of sperm, and morphologically abnormal sperm by the methods described previously [9]. As no sperm had been found in the semen of either Dog 1 or Dog 2 (Table 1), both of them were diagnosed as having azoospermia. Although the two azoospermic dogs were treated with 3 intramuscular injections of 1000 IU hCG per head at one-week intervals, their semen quality had failed to improve. Peripheral blood samples were collected from the two azoospermic dogs at the same time as the semen collections. Plasma E 2 and T levels were measured by the radioimmunoassay described previously [8]. The plasma E 2 levels of the two dogs (21 and 22 pg/ml, respectively) were higher than those o...

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