Summary. Testis mass of adult Cape mountain zebra stallions (mean 70·0 g) was appreciably less than that of other zebra species and domestic horses. The histological appearance of the testes of 11-, 24- and 29-month-old colts was typically prepubertal. Spermatogenic activity of a 4-year-old stallion obtained at the end of summer was at a very low level, while a 4·5-year-old stallion obtained 6 weeks after the winter solstice showed a marked increase in spermatogenesis compared with the 4-year-old. Stallions 6·5–19 years of age collected in different seasons all showed active spermatogenesis.