Apodemus agrarius，the main pest rat in China，represents 94.81% of all farmland rats in Yuqing County，Guizhou. From 1987-2005，we captured rodents once a month (from 5th-15th day of the month) in rice field and dry field habitats. Captured A. agrarius were brought back to laboratory，numbered，measured for morphological characteristics，and then autopsied to determine sex，age and reproductive condition.
A total of 5497 specimens were captured，among which 2698 were male and 2799 were female. The rats bred from January to November，and the most favorable breeding period was March to October. Two reproductive peaks (April-May and August-September) were evident. Sex ratio (♀/♂)，the average pregnancy rate，mean litter size，reproductive index and the rate of scrotal testis presence for the whole studey population were 0.96，36.91%±6.79%，5.33±1.07，0.81±0.17 and 59.13%±7.89%，respectively.
Individuals were assigned to five age classes based on either weight or body weight without viscera. Reproductive data of 1261 A. agrarius from 1999 to 2005 showed obvious variation with the age. All juveniles were sexually immature with no reproductive activity.The sub-adult group had begun to reproduce with a pregnancy rate is 4.68% and a rate of scrotal testis presence of 14.97%. All of the adult group I，group II and the old group were sexually mature，with a pregnancy rate ranging from 31.62%-68.33% and a rate of scrotal testis presence from 79.39%-100.00%. Reproductive capacity would increase continuously with growth within age categories. Adult group I，group II and the old group were the main reproductive contributors.
Comparison of reproductive parameters in A. agrarius captured from 1987 to 2005 showed that the pregnancy rate，mean litter size，the rate of scrotal testis presence and the reproductive index (except for sex ratio) varied by month and season. However，in different years，only the sex ratio showed significant variation. Reproductive parameters of A. agrarius were relatively stable among different years，but fluctuated in different months and different seasons. Correlational analysis of reproductive parameters showed that population densities each year were positively correlated significatively with the reproductive index and mean litter size，whereas there was no correlation between populations and either the pregnancy rate，the rate of scrotal testis presence，or the sex ratio. Population density of A. agrarius depends strongly on the reproductive index，which is very useful in forecasting population density for the rodent.
Comparison of reproductive parameters of A. agrarius among different areas in China demonstrated obvious regional variation. Mean litter size and reproductive capacity increased gradually from south to north；i.e.，the rat’s fecundity increased with increasing latitude. However，reproductive period was lower in high-latitude areas. Sex ratio increased from south to north and also from west to east. Reproductive parameters were not correlated with altitude，so this parameter can be eliminated in analysis of the reproductive traits of A. agrarius.