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Male and female C57BL/Tw mice were given 5 daily subcutaneous injections of 100 micrograms tamoxifen (Tx), starting on the day of birth (Tx mice). In untreated fetal mice on day 18 of gestation, the greater part of the pubic and ischial bones were cartilaginous. At more than 30 days of age, however, untreated mice showed completely calcified pelvic bone, whereas in age-matched Tx mice the greater part of the junctional regions in the pelvis remained cartilaginous. Treatment with Tx starting within 5 days of age caused bladder hernia with or without cecum hernia. The pubic ligament in Tx mice at ages of 30-540 days was markedly expanded as compared with that in age-matched controls. The permanent chondrification in the pelvis was found in all mice given Tx starting within 10 days of age. By contrast, neonatal treatments of mice with other antiestrogens, clomiphene and nafoxidine (100 micrograms/day), induced neither permanent chondrification in the pelvis nor expansion of the pubic ligament nor hernia. These findings suggest that Tx has a specific effect on the symphysis pubis and some junctional regions of the developing pelvis in mice when given neonatally.