The amniotic fluid lecithin/sphingomyelin (L/S) ratio in normal rhesus monkey pregnancies exhibits a distribution through the latter half of gestation similar to that seen in human pregnancies. Changes in the synthesis and concentration of lecithin in the fetal lung, measured both in vitro and in vivo, are paralleled by changes in the amniotic fluid L/S ratio. Both the amniotic fluid L/S ratio and fetal lung lecithin concentration increase significantly (p less than 0.001) in the final 10 per cent of rhesus monkey gestation, and there is a significant correlation (p less than 0.001) between these two indices of fetal lung lecithin metabolism. Moreover, the onset of these late gestational changes is temporally related to increased activity of the major pathway of de novo lung lecithin synthesis and to the ability of the preterm rhesus newborn infants to remain free of respiratory symptoms after delivery by cesarean section. We conclude that the amniotic fluid L/S ratio is a valid indicator of fetal pulmonary phospholipid metabolism and, therefore, an accurate index of biochemical pulmonary maturity.