Acetylcholinesterase (AchE) and butyrylcholinesterase (BuchE) activities were studied in 20,000 g supernatant (S) and pellet (P) fractions of cerebrum1, caudate nucleus and cerebellum regions of human brain between the gestational ages of 20 and 40 weeks. On a per gram wet tissue basis, cerebrum and cerebellum had 60% and caudate nucleus had 80% of AchE activity present at 20–22 weeks as compared to 38–40 weeks of gestation. Specific activity (SA) of AchE did not change appreciably with gestational age in any of the P fractions. In S fractions of cerebrum and cerebellum there was about a 1.5-fold increase during this period. BuchE SA and activity per gram tissue in P and S fractions of cerebrum and cerebellum increased by 1.5- to 3-fold and decreased by about the same order of magnitude in caudate nucleus, during 20–40 weeks of gestation.