The hypothesis that estrogens are tumor promoters was tested by tumor induction in the rat vagina. Ovariectomized rats were given a single dose of N-methyl-N-nitrosourea (MNU) by vaginal instillation, followed one week later by long-term release Silastic implants containing estradiol (E2). After 16 months, a significant incidence (4/9) of benign vaginal stromal polyps was found in the MNU-E2 group, but no vaginal polyps were seen in groups given either MNU or E2 alone. A number of non-neoplastic changes were also seen and were due to E2 treatment either with or without MNU. The incidence of stromal polyps and their restriction to animals receiving the initiator-promoter regimen alone suggests that estrogens promote tumorigenesis in the rat vagina.