Effect of housing stress on the formation and development of tumors in rats.

Tumor induction by 7,12-dimethylbenz[a]anthracene (DMBA) or inoculation with adenocarcinoma cells was studied in rats raised in groups (group G) or individually (group I) as well as in rats raised in groups and switched to individual housing at the beginning of the experiment (group G/I). The number of DMBA-induced tumors did not differ among the three groups of female rats but tumor weight was lowest in group I, highest in group G/I and intermediate in group G. The mean weight of adenocarcinoma cell-induced tumors was highest in group G/I and did not differ between groups I and G. Male rats generally showed higher tumor weights than females, the values becoming significant in group G/I. The results show that housing and sex can significantly affect tumor growth.