The internal organs of small mammals show phenotypic plasticity. The relative masses of organs may be affected by an animal's growth and development, by breeding and seasonal cycles, or by habitat change. Organ masses might therefore serve as an indicator of the responses of animals to environmental conditions or to social stress. It is well-known that population density influences growth, development, breeding, and behavior of rodents but little has been reported about density-dependent effect on internal organs. The Yangtze vole (Microtus fortis calamorum), a small herbivore species on the beaches of Dongting Lake, in Hunan Province, China, is an important rodent in this region. During the flood season, the voles escape flooded beaches by migrating across dikes to the surrounding rice fields, where they can cause serious damage to crops. This study investigated the influence of rearing density (high density as a social stressor) on the internal organs (heart, lung, liver, spleen, kidney, and adrenal gland) of the Yangtze vole. A total of 258 adult voles (sex ratio 1:1) were randomly divided into 4 groups and reared at densities of 2, 4, 6 and 8 animals per cage. After 90 days, the masses of the internal organs were measured and the plasma cortisol concentration was determined by radioimmunoassay. Differences of organ masses at different densities were analyzed by ANCOVA with carcass mass as the covariate. Two-way ANCOVA indicated that liver mass was significantly different in both density and pregnancy condition. One-way ANCOVA revealed a very significant difference in liver mass of pregnant females at density of 4 per cage, while there were no significant differences in the masses of other internal organs between pregnant and non-pregnant voles. Except for the density-dependent effect on the liver of pregnant voles, there was no significant effect of density on the masses of any other internal organs, in either pregnant or non-pregnant voles. Two-way ANCOVA showed significant sex difference in the masses of the spleen and adrenal glands, and density-dependent of the masses of the lung, spleen and adrenal glands. One-way ANCOVA showed that mean spleen and adrenal glands mass of males was lower than that of females. One-way ANCOVA also revealed significant differences among different densities in the masses of the lung and spleen in males, and in the masses of the spleen and adrenal glands of females. No differences were observed between the sexes, or among different densities, in the masses of other organs. Although other internal organ masses showed no significant differences among densities, they all gradually increased with increasing density of population. The plasma cortisol concentration was clearly higher in males than in females within each density group. Among males, there were significant differences in plasma cortisol concentrations at different densities, but not in females. Taken together, our observations of density-dependent effect support the concept of plasticity of internal organs in response to social stress, although population density influenced different organs differentially. However, further research must be conducted into the effects of density on reproductive characteristics and organ mass.