SUMMARY

The three experimental groups consisted of mature females maintained singly, in groups of six, or grouped in threes with a vasectomized male. The animals were inbred from wild house mice. Duration or frequency of oestrus did not differ between the isolated and the grouped females at 10 weeks of age, but at 20 weeks of age the mean duration of oestrus was greater in the isolated females. Degeneration of the adrenal X-zone and an increase in the number of corpora lutea in the ovaries occurred with grouping. These changes were enhanced in the females which had undergone recurrent pseudopregnancies. In these latter animals, mammary acinar proliferation and uterine endometriosis were seen. Alterations in the histology of the adrenal glands and ovaries, together with characteristics peculiar to this strain, are described. The degenerated X-zone, increased corpora lutea, uterine changes, and mammary acinar development, suggest that there may be an increase in androgens and progestins with grouping and pseudopregnancy.