Organophosphate and carbamate pesticides inhibit acetylcholinesterase (AChE) at nerve synapses. Blood samples from 22 Hispaniolan Amazon parrots (Amazona ventralis) were assayed for cholinesterase levels by two different techniques. Using the modified Michel method, the whole-blood cholinesterase activity levels ranged from 0.082 to 0.616 ΔpH/hr with a mean value of 0.35 ΔpH/hr. A reference range (0.08–0.62 ΔpH/hr) for cholinesterase was established in birds. The modified Ellman spectrophotometric method was used to measure AChE activity by adding acetylthiocholine or pseudocholinesterase (plasma cholinesterase) activity by adding butyrylthiocholine. The reference range of the AChE activity using the modified Ellman spectrophotometric method was 0–1.12 μmol/ml/min with a mean of 0.48 μmol/ml/min, and for pseudocholinesterase the range was 0.09–0.98 μmol/ml/min with a mean of 0.53 μmol/ml/min.