Although capybara is the largest rodent in the world and largely distributed in Central and South America, there is no report in the literature concerning the cycle of seminiferous epithelium in this species. In the present study, the length of spermatogenic cycle was estimated using intratesticular injections of tritiated thymidine. Animals were sacrificed at 1 h, 8 days, and 17 days after injections. The duration of one spermatogenic cycle in capybaras is 11.9 +/- 0.1 days (SEM). Spermatogenesis was estimated to last 53.6 days, when considering that the total duration of spermatogenesis takes about 4.5 cycles of seminiferous epithelium. The approximate life span of primary spermatocytes is 19.1 days, while spermiogenesis lasts 16.7 days. Staging in capybaras was based on the spermatid nuclei shape and location of spermatids, named tubular morphology method, which consists of 8 stages in all species. The relative stage frequencies in capybaras, based on the analysis of approximately 200 cross sections of seminiferous tubule for each of the ten animals were as follow: stage 1: 14.0 +/- 1.5%; stage 2: 15.1 +/- 1.0%; stage 3: 15.7 +/- 1.1%; stage 4: 14.6 +/- 1.1%; stage 5: 8.7 +/- 0.7%; stage 6: 7.0 +/- 0.7%; stage 7: 9.4 +/- 0.9%; stage 8: 15.5 +/- 1.0%. The pre-meiotic, meiotic and post-meiotic phases relative frequencies were 44.8%, 14.6% and 40.6%, respectively. Compared to most rodents investigated so far, the duration of spermatogenesis in capybaras is relatively long.