Acetylcholinesterase (AChE) activity was detected in amniotic fluid either sampled by amniocentesis at 14-23 weeks gestation or collected at normal parturition. Activity ranged from 0·2 to 8·9 u/l in the earlier samples and from 0·2 to 4·2 u/l in the later samples. Since the fetal but not the maternal serum contained AChE activity, we suggest that the AChE in the amniotic fluid is derived from the fetus. AChE is released from both neural and non-neural tissues, and the release of the enzyme from both sources may diminish as the nervous system matures. The amount of AChE found in amniotic fluid may be affected by abnormalities of the nervous system. Amniotic fluid from two fetuses with spina bifida contained AChE activity: one, with a gestational age of 21 weeks, was in the upper levels of the normal range (4·5 u/l), whereas the other, at 36 weeks, contained 1·9 times more activity than the next highest sample. In a 20-week fetus with anencephaly, the AChE activity in the amniotic fluid was increased 2·8-fold over the activity of the highest "normal" sample.