Dilatation of the uterine cervix at parturition is accompanied by a remarkable alteration in its biomechanical characteristics leading to a significant reduction in its strength and stiffness. Our previous studies and those of others have suggested the involvement of collagenolysis leading to cervical dilatation. This study provides further evidence for the occurrence of collagenolysis in the dilated guinea pig cervix at birth. The changes in collagenase and collagenase inhibitory activity in vivo in cervices of nonpregnant, pregnant, and postpartum guinea pigs were determined. There were no significant changes in procollagenase, collagenase inhibitory activity, and net procollagenase in animals at 25 and 50 days of gestation compared with nonpregnant animals. At parturition (68 +/- 2 days), there was a 6-fold increase in procollagenase, a 26-fold increase in collagenase inhibitory activity, and a 2-fold increase in net procollagenase activity. Cervices in organ culture obtained at birth produced 2.9-fold more procollagenase, 1.6-fold more collagenase inhibitory activity, and a 10-fold increase in net procollagenase activity when compared to nonpregnant or 25-day pregnant animals. These studies indicate that dilatation of the guinea pig cervix at parturition involves collagenase-mediated degradation of collagen in the cervix.