Cervical ripening in gestation occurs spontaneously in a timely but species specific fashion as if it were genetically programmed. This study shows that the numbers of dying smooth muscle cells in the cervix increased along with cervical softening in pregnant rats from day 12 of gestation to day 21. The morphological characteristics of the chromatin cleavage in apoptosis are identified. Internucleosomal DNA fragments in the DNA preparations as well as the ladder pattern of double-stranded DNA observed on gel electrophoresis are observed. The deletion and decrease of the cell population in the cervices was demonstrated by a decrease of total DNA in these tissues. The interesting possibility is raised that programmed smooth muscle cell death in the cervix may be involved in the processes of the cervical ripening.