Trypsin activity is monitored in a simple kinetic method with an ammonia gas sensor. The method is based upon the hydrolysis of α-benzoyl-arginine amide resulting in the production of ammonia. Day-to-day precision range of ±1.9% to 4.4% and a within-run precision range of ± 1.8% to 4.2% were measured for the working range of 0.5 to 20 μg mL−1trypsin. An average of 97 ± 3.4% added trypsin was recovered from rat pancreatic juice in recovery experiments.