Serial samples of amniotic fluid were taken from 38 patients in spontaneous labor and the lecithin:sphingomyelin (L:S) ratios were estimated. The fetal heart rate (FHR) pattern for each patient was analyzed, and the baseline rate, long-term variability, and periodic changes wee determined. It was found that changes in the L:S ratios were significantly correlated with the duration of labor and FHR long-term variability. Very low L:S ratios were observed in some patients whose infants did not subsequently develop respiratory distress syndrome. This raises the possibility that during labor the amniotic fluid is no longer representative of lung fluid, perhaps because of reduced tracheal fluid efflux, a process which may operate when the fetus is under stress.