Changes in reproductive parameters were analyzed quantitatively in a natural population of the small Japanese field mouse, Apodemus argenteus. Among individuals born in the current year, the lightest female weighed 8 g at sexual maturity and the lightest male weighed 10 g. Irrespective of season, the lightest mice were found only during the phase of population increase. Among mice that had over-wintered, the lightest individuals of each sex to reproduce during the spring population decline weighed 10 g. In years of high population density, the reproductive rates of females, even at their peak during August, was below 10%. In contrast, in low-density years, much higher rates, over 69%, lasted until October during the increase phase. The patterns for males were almost the same as those of females during survey years. According to multivariate analysis, the reproductive rate of males was largely explained by population density (partial correlation, 0.732), whereas the reproductive rate of females was largely explained by the fluctuation phase (0.848). The number of each sex to reproduce increased in proportion to the density of potentially reproductive mice at lower densities, but then decreased at higher densities. The observed maximun number of reproductively active mice was 15 males and 21 females in a one-hectare grid. Temperature appeared not to cause any variation in the breeding season in this population.

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