A study was conducted to evaluate the associations among testis size, testosterone concentrations, semen parameters and aspects of spermatogenesis in Nelore bulls (n = 28). Testis size was measured from 10 to 29 months of age. Bulls were treated with GnRH (12 to 21 months) and semen samples also collected from 25 to 29 months. At 30 months, animals were slaughtered. Correlations were significant when p < 0.05. Basal testosterone was highest at 18 months, suggesting that bulls reached puberty at this age. At 30 months, seminiferous tubules represented 77.9 ± 3.8 % of the testicular parenchyma and there were 33.6 ± 8.4 round spermatids/A1 spermatogonium/tubule section, indicating a 47.5% degeneration rate during spermatogenesis. At 30 months, heavier testis correlated with Sertoli cell numbers/testis (r = 0.77), and round spermatids/tubule section, Sertoli cell and A1 spermatogonium (r = 0.50 – 0.60). Scrotal circumference (SC) taken between 10 and 29 months correlated with the percentage of tubules with spermatids (r = 0.42 – 0.59) and number of A1 spermatogonium and round spermatids/Sertoli cell (r = 0.49 – 0.68). Epididymal weight was related to Sertoli cell numbers/testis, and round spermatids/ Sertoli cell and A1 spermatogonium (r = 0.51 – 0.61). GnRH-stimulated testosterone from 17 to 21 months correlated with SC between 14 and 29 months (r = 0.48 – 0.60), testis and epididymal weights (r = 0.41 – 0.64) and with parameters of spermatogenesis (r = 0.44 – 0.58). Additionally, sperm motility and vigor from 25 to 29 months correlated with the number of tubules with spermatids (r = 0.42 – 0.59) and GnRH-stimulated testosterone at 12, 13 and 18 months (r = 0.46 – 0.57). In conclusion, testis size during and after the period of pronounced increases in testosterone is an indicator of quantitative parameters of spermatogenesis of post-pubertal bulls.