Amniocenteses were performed upon gravidas ranging from gestational weeks 29—43 to determine the amniotic fluid osmolality of normal and abnormal pregnancies. The osmolality decreased with advancing gestational age and continued to do so after the estimated term date. An amniotic fluid osmolality level of 250 mOsm/liter or less is suggestive of fetal maturity. Amniotic fluid osmolality may have potential value in predicting the onset of labor.