Composition of the amylase isozymes in the amniotic fluid and their transition with fetal age were studied by means of thin-layer acrylamide gel electrophoresis. The results indicate that amylase isozymes in the amniotic fluid were composed of a pancreatic type faint major isozyme and several salivary type isozymes consisting of an intense major and a few less intense minor bands. Characteristic changes in the pattern of these isozymes with fetal age consisted of a marked increase of the salivary type isozymes rather than of the pancreatic type isozyme. A densitometric analysis of the isozyme patterns proved that the ratio of the pancreatic type isozyme to the salivary type major isozyme was highly correlated with fetal age regardless of levels of amylase activity.