Summary. Rats were considered to be pubertal at 50 days of age when spermatozoa were first found in the tail of the epididymis. Sperm production/g testis increased up to 75 days of age and testicular weight increased until 100 days of age. Sperm reserves in the tail of the epididymis were not maximal until 100 days of age. Therefore, Wistar rats are not sexually mature until 100 days. Sexually mature rats had testes weighing 3·7 g, produced 86 × 106 spermatozoa or 24 × 106 spermatozoa/g testicular parenchyma daily, and their paired epididymides contained 295 × 106 spermatozoa in the head + body and 440 × 106 spermatozoa in the tail.