The effect of in utero exposure to cocaine on the developmental pattern of acetylcholinesterase (AChE), a major regulator of the central nervous system neurotransmitter acetylcholine, was studied in fetal brain cell cultures collected from mice on gestational day 15 after maternal exposure to cocaine from gestational days 6-14. A significant decrease in total and specific activity (expressed per mg of protein) of acetylcholinesterase was seen throughout the culture period in the cells grown from the cocaine-exposed animals as compared to controls (p less than .002). Similarly, the total protein content of the brain cells grown from the cocaine-exposed animals was significantly decreased as compared to controls (p less than .03). Utilizing AChE as a biochemical marker, these studies have shown that in utero exposure to cocaine has an adverse effect on the normal developmental pattern of the production of acetylcholinesterase in cholinergic neurons in the fetal mouse brain.