Employing a hydrogen gas clearance method, cervical blood flow abbre . CxBF hereafter, was measured in non-pregnant and various pregnant stages and the following results were obtained. CxBF in 19 non-pregnant cases was 54.1 +/- 22.0 ml/min/100 gr. The measurement was 63.1 +/- 38 in the early, 54.5 in the late proliferative, 65.2 +/- 17.1 in the early and 25.3 +/- 8.9 in the late secretory stage, suggesting that the measurement of CxBF was effected by hormonal conditions. No relationship was found between uterine weight and CxBF . CxBF was 96.8 +/- 11.3 in 6-7, 48.6 +/- 26.4 in 8, 55.5 +/- 9.8 in 9-13 gestational weeks, that is it was highest in the early stages and gradually decreased with the advance of pregnancy. CxBF was considered to be affected by the physical distance between the cervix and the site of the placenta of the chorion frondosum . CxBF was 22.5 +/- 10.8 in and after the 37th gestational week and the value significantly decreased in the terminal stage of pregnancy (r = -0.758, p less than 0.01). The decrease in the terminal stage is supposed to reflect the circulatory insufficiency, that is, cervical ripening. CxBF in molar pregnancy was higher than in normal pregnancy.