Radioimmunologic determination of free cortisol in amniotic fluid of 150 patients, was carried out; 117 women were not in labor, 21 were in labor, 12 were for cesarean section without labor, in the patients without labor cortisol varied during the time studied (31-42 weeks) without statistical significance. Cortisol in the amniotic fluid increased significantly in patients in labor as compared with cesarean section patients not in labor; this increase probably is due to fetal stress during labor.