Plasma acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activity was measured repeatedly in 27 mallard (Anas platyrhynchos) ducklings between 7 and 85 days of age to determine age-dependent changes in enzyme activity. Plasma AChE, BChE, and total cholinesterase (ChE) activity decreased significantly with age. The relative proportion of AChE in total ChE activity also decreased slightly with age. Since some anti-ChE chemicals can selectively inhibit AChE or BChE activity, characterization of age-dependent changes in the activity of each enzyme may be necessary to accurately identify the occurrence of pesticide exposure.