Abstract: The purpose of this study is to determine experimentally the influence of population density on reproduction in female mice under different sexual ratios.168 mice at 28 days of age were selected at random and either placed in groups of 40 or 8 mice per cage to create 6 populations of high and low density.Within each group the populations differed in the proportion cf sexes, i.e.the ratios of females to males were either 1:1,3:1 or 1:3.All females were inspected between 8 and 11 AM for vaginal opening and after vaginal introitus,a daily vaginal smear was taken en each mouse to determine the age at first estrus.Each mouse was weighed daily.Treatment continued for 58 days.Half of pregnant females at 58 days of age remained to cage singly till parturition,while the others were sacrificed.Each mouse was weighed ard autopsied,and the internal organs were removed,preserved in 10% formalin and then dissected,cleaned out fat and weighed to the nearest 0.1mg on a analytical balance.After parturition,litter size was checked ard the data on growth and development of young mice were collected. Physiological and morphological comparative studies were made of high and low populations with different sexual ratios.The later will be discussed in other paper.Analysis of data for vaginal perforation,first vaginal oestrus,oestrus cycle showed existence of differences between populations of various densities and various sexual ratios. Delayed sexual maturation and decreasing reproductive rate were found in populations of high density,especially in populations with bias of the sex ratio in favor of females. Regression analysis showed that age at first oestrus was related to increasing rate of body weight. Delayed sexual maturation was accompanied by a slow increase in body weight.percentage of pregnancy is inversely proportional to the population density. The lowest percentage of pregnancy were recorded in high population with sex ratio of 3:1.The average litter size showed ro significant diffeiences related to density, but to bias of the sexual ratio in favor of males.The decreasing rate of survival of juvenile mice from population with sex ratio 1:3 was related to aggressive behavior of adult male mice in low population.It is concluded that effect of population density on reproduction in female mice changed under different sex ratios. Inhibition of reproduction in females occured in all populations of high density, especially in those populations with sex ratio of 3:1.The important role of social factors on reproduction and survival in mice were also discussed.