Abstract: The present report is a part of our study on the ecology of the rat, Rattus rattoides Hodgson, The data of reproduction were based on 7471 rats which were captured in fields during 1963-1966, The authors attempt to give a brief analysis of the relations between the state of reproduction and the changements of population density. The results may be briefly summarized below.1. The main factors which lead to the changes of population density are not the number of the rats, but the length of reproduction period and the pregnant rate.2.The annual average pregnant rate was 30-42%,the peak of which was in autumn (72-92% in October of 1963, 72-41% in October of 1964, 68-18% in September of 1965), So that the population density took its peak values in winter of the same year and the coming spring.