Proliferation of the vagina and uterus in rodents is stimulated by ovarian estrogen, and ovariectomy induces regression of these tracts. In mouse, reproductive tracts, organ weights, vaginal epithelial thickness, the number of epithelial cell layers, and uterine epithelial cell height were significantly reduced 1 to 20 days after ovariectomy when compared with those of intact, mice taken at the estrous stage. No significant differences in those parameters were found between ovariectomized mice implanted with estradiol and intact estrous mice. Vaginal epithelium at estrus was composed of 10-15 layers of epithelial cells with superficial keratinized layers, however, the vaginal epithelium began to regress one day after ovariectomy. Infiltration of leukocytes was encountered in the vaginal epithelium 2-3 days after ovariectomy. Protein expression in the regressing vagina and uterus after ovariectomy was examined by two-dimensional polyacrylamide gel electrophoresis. After ovariectomy, new proteins appeared in the regressing vagina, but not in the regressing uterus. Expressions of 7 and 6 proteins in the vagina and uterus, respectively, reached peak levels 2-3 days after ovariectomy. The expression of ovariectomy-specific proteins and infiltration of leukocytes in vaginal epithelium was inhibited by the injection of cycloheximide, an inhibitor of protein synthesis. These results suggest that some proteins participate in the regression of mouse reproductive tracts after estrogen withdrawal induced by ovariectomy.