Changes in the activity of beta-N-acetylglucosaminidase (E.C. 3.2.1.30; NAG), a lysosomal marker enzyme, in the maternal plasma and utero-placental tissues during late pregnancy and parturition were examined in order to determine if lysosomal enzymes participate in the cervical ripening. NAG activity in the plasma gradually increased as gestation advanced, to reach a maximum 3-4 days before the spontaneous onset of labor, and remaining at this level until the second stage. The activity measured in decidua and amnion obtained from the women who had undergone routine repeat cesarean section after labor onset was lower than in those before the onset. Since the decidua was much heavier than the amnion, it was concluded that NAG was released to the maternal circulation mainly from the decidua and only partly from the amnion. Our data revealed that lysosomal enzyme release occurs prior to the onset of labor, and that NAG may play a role primarily in the ripening of the cervix rather than in the onset of labor, by hydrolysing GAGs in the uterine cervix.