The purpose of this study was to establish I) the sequential changes in specific activity of phosphatidic acid phosphohydrolase (PAPase) in amniotic fluid and its relation to the Iecithin/sphingomyelin (L/S) ratio; and 2) the origin of amniotic fluid PAPase. The increase in the amniotic fluid L/S ratio is preceded by an increase in PAPase activity, rising from 15 nmoles of phosphate released per milligram of protein per hour at 30 necks la 101) nmoles at 37 weeks. The mean PAPase activity in the nasopharyngeal fluid of the infant is 456 nmoles of phosphate released per ml per hour, the amniotic fluid mean PAPase activity at delivery being 129 nmoles ( P<0.01). These findings are consistent with the view that amniotic fluid PAPase originates, in part, from the fetal lung and likely participates in the regulation of the synthesis of lecithin.