It is important to know the optimal timing of mating to produce laboratory animals efficiently and to ensure the preservation laboratory strains. To clarify the characteristics of the male sexual maturity of two laboratory Mictorus strains, namely the “Mar” and “MrosA” strains, which are derived from M. arvalis and M. levis, respectively, the testes and epididymes from 4-, 6-, 8- and 10 week-old males were weighed and compared. Although the tissue weights increased gradually with age in the Mar strain, in the MrosA strain, the weight of the testis increased markedly from 4 to weeks of age and that of the epididymis increased from 6 to 8 weeks of age. In addition, the spermatozoa concentrations in the cauda epididymis at 6, 8, and 10 weeks of age in were measured in each strain. The sperm concentration increased in the Mar strain at 8 weeks versus 6 weeks in the MrosA strain. From these results, it is clear that there are differences in the patterns of the increases in testis and epididymis weight and sperm concentration between the two strains. It was suggested that males of the MrosA strain attain sexual maturity earlier than those of the Mar strain.