Changes in interstitial collagenase activity in the rat uterine cervix during ripening were clarified in a time-dependent manner. Premature delivery was induced by an antiprogesterone agent, RU486, for rats in late pregnancy. The presence of interstitial collagenase in the extract from the rat cervical tissue was demonstrated, by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis using the natural and unaffected collagen as a substrate. The collagenase activity was determined as the release of digested peptides from the radio-labeled collagen. Our experiments with RU486 were performed in rats on the 18th day of pregnancy. A single administration of RU486 (15 mg/kg) resulted in the premature delivery of all treated rats within 30 h after the injection (average time was 23.9 h). The marked increase in cervical wet weight was observed up to the time to premature delivery along with a significant acceleration from 18 h after the adminsitration of RU486. In this state, the cervical collagenase activity was enhanced, the highest levels being recorded at 21 h after the administration. The interstitial collagenase in the uterine cervix appears to play a significant role in the regulation mechanisms of cervical ripening in late pregnant rats.