ABSTRACT: Dimethandrolone undecanoate (DMAU: 7α,11β-dimethyl-19-nortestosterone 17β-undecanoate) is a potent orally active androgen with progestational activity that is in development for therapeutic uses in men. We hypothesized that because of its dual activity, DMAU might have potential as a single-agent oral hormonal contraceptive. To test this possibility, adult male rabbits (5/group) of proven fertility were treated orally with vehicle or DMAU at 1.0, 2.5, 5.0, or 10.0 mg/kg/d for 12 or 13 weeks. Semen and blood samples were collected every other week through week 30. Sperm were decreased (P < .05) in semen samples from DMAU-treated rabbits at 2.5 and 5.0 mg/kg/d at weeks 12, 14, 16, 18, and 20 compared to week 0 (prior to treatment). The percentage of forward progressive motile sperm in those rabbits that still had measurable sperm was also reduced by DMAU treatment at 2.5 mg/kg/d at weeks 14, 16, 18, and 20 and at 5.0 mg/kg/d at week 18 (P < .05). At 1.0 mg/kg/d only 1 rabbit had reduced sperm numbers and motility. A mating trial was performed at week 15. The number of bred males that were fertile was 4 of 4 in the vehicle-treated group and 4 of 5, 0 of 4, and 2 of 5 in the 1.0, 2.5, and 5.0 mg/kg/d DMAU treatment groups. By week 22, sperm numbers and forward progressive motility increased, and they returned to pretreatment levels in all DMAU-treated rabbits by week 30. All bred males were fertile at week 31. Serum levels of testosterone, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) were significantly suppressed in DMAU (1.0, 2.5, or 5.0 mg/kg/d)-treated rabbits during the 12-week dosing interval, but were comparable to pretreatment levels after cessation of dosing. These data indicate that DMAU suppressed the hypothalamic-pituitary-gonadal axis, resulting in severe oligospermia in the majority of rabbits in the 2.5 and 5.0 mg/kg/d dosing groups. Infertility was observed when sperm numbers decreased to about 10% of pretreatment levels. In rabbits dosed with DMAU at 10.0 mg/kg/d, no effect on sperm numbers or motility was observed by week 12. Dosing continued for another week, and the rabbits underwent a gross necropsy on week 13 with removal of testes and epididymides for histology and preparation of testicular cytosol. Serum testosterone, FSH, and LH levels were considerably suppressed in these rabbits as in the lower-dose groups. The lack of oligospermia in the 10.0 mg/kg group as well as in the 2 fertile males in the 5.0 mg/kg group may have been due to high intratesticular levels of 7α,11β-dimethyl-19-nortestosterone, the active metabolite of DMAU. Hence, as observed previously for testosterone, DMAU has a biphasic effect on spermatogenesis. Collectively, these data indicate that DMAU has the potential to be an orally active single-agent male hormonal contraceptive at an appropriate dose level and should be tested for contraceptive efficacy in nonhuman primates.