Abstract: Observations on reproductive intensity，duration of breeding season，rate of maturation and age structure were made in both high-density and low-density populations of Brandt's vole (Microtus brandti)．Samples were collected simultaneously in the two populations in Taibus Banner，Inner Mongolia,1987. The results showed that population density as a factor affecting the population characteristics and regulating the popution dynamics of Brandt's vole.
In the high-density population，the reproductive intensity was suppressed．In July, percentage of pregnant females (77.27% )，males with descendant testes(33.33%)，males with enlarged seminal vesicles (22.22% ) and testis Iength(8.52mm)in the high-density population were less than those in the low-density population (88.06% ，49.23%，38.46%and 11.49mm respectively)．Breeding season ended earlier in the high-density population than that in the low-density population．In the high-density population，percentage of pregnant females was only 3.45%and percentage of males with enlarged seminal veslcles was1.41% in August while they were 13.79 %and 5.41%，respectively，in the low -density population．It showed that duration of breeding season in high density population was shorter than that in the low-density population．
Sexual maturation of the youngs was suppressed in the high-density population．In July，percentage of pregnant females in age classⅡ of the low-density population (94.74%) was higher than that in the high-density population (60.00% )．And 71.43% of females in age classI wsre pregnant in the low-density population while there was
noany pregnant femals in the same age class of the high-density population．
There was difference in age structure between the two populations． In August,only 8.86% of population were of age class I and 26.58％ of that were of class II in high-density population whill in low-density population they were 17.42％ and 12.88%, respectively.Therefore the population of the youngs in the high-density Populauion was less than that in the low-density population．
There wasno significant difference in relative fatness between the two populations, It suggests that the two populatins share the same nutritional condition．So，the factor which causes suppression of reproduction in the high-density populatlon is social factor of high-density rather than shortge of food resources．The fatness in age class I of the high-density populatlon was less than that of the low-density population．It might due to social stress of high density resulted in worse parental care．

Key words: Brandt's VOle(Microtus brandti), Populadon density, Reproduction, relative fatnss

摘要： 对布氏田鼠栖息密度不同的种群同时进行取样，研究密度因素对布氏田鼠种群发展的调节作用。结果表明，在高密度医布氏田鼠种群繁殖强度受到抑制，雌鼠怀孕率、雄鼠睾丸下降率、贮精囊肥大率和睾丸长度都小于低密度种群。高密度种群繁殖季节结束时间早于低密度种群，幼鼠肥满度较小，性成熟速度较慢，种群年龄结构中，幼年鼠所占比例小于低密度种群。这些都是导致高密度种群增长速度减慢的因素。

关键词: 布氏田鼠, 种群密度, 种群调节, 繁殖, 肥满度