The number of spermatogonia per tubular transverse section (S/T), and the percentage of seminiferous tubulus containing spermatogonia (the fertility index (FI] were measured in 40 pairs of normal autopsy testes aged 28 weeks of gestation-40 years. S/T and FI showed similar changes during the whole period, and were minimal between 1 and 4 years. The number of spermatogonia per testis (S/testis) and the number of spermatogonia per cm3 testis tissue (S/cm3) were estimated by stereological methods in the same testes. S/T and FI respectively were significantly correlated both to S/testis and S/cm3. So quantification of spermatogonia by S/T or FI measurement is a reasonable alternative to stereological examinations.