The concentration of amylase in amniotic fluid during the last half of pregnancy was studied. Single determinations were done in 180 patients and serial determinations in 7 patients. The results have been correlated with gestational age and fetal weight. Amylase concentration increased very gradually up to approximately 36 weeks and after this time abrupt increases were observed. When the amylase level is 200 I.U. per liter or greater, the correlation with a mature fetus is very good. However, values less than 200 I.U. per liter are associated with both mature and immature infants. No correlation was found between amniotic fluid and maternal serum amylase levels.