Two adult male gerenuk (Litocranius walleri walleri) were confirmed infertile with distinctly varying clinical presentations. One animal had unilateral testicular degeneration/hypoplasia, and within 8 mo experienced atrophy/degeneration of the remaining testicle. The second animal had been previously treated with melengesterol acetate (MGA) milled in feed for 1 yr during puberty as part of an aggression-control study. The testes in this individual appeared normal both visually and on palpation; however, repeated semen collection consistently produced ejaculates containing high numbers of immotile spermatozoa, all of a single abnormal morphology: shortened tails, with normal total sperm counts for this species. Both gerenuk had cortisol concentrations within normal ranges for adult male gerenuk. Analysis of serum mineral concentration revealed zinc levels that would be considered low in domestic cattle. Testosterone levels were low for the animal discussed in case 1, but were within normal range for the animal in case 2 compared with other gerenuk. Investigations into endocrine causes, such as abnormal thyroid hormone concentrations and adrenal function, were unrewarding. Both animals discussed in this report are maternally related; therefore, a genetic cause of infertility cannot be ruled out. Further investigation into MGA, as well as the dietary zinc requirements for gerenuk, and resulting effects on spermatogenesis and testicular development are warranted.