Tryptic hydrolysis of different arginine nitroanilides was studied. Para-nitroanilide is the most readily hydrolyzed substrate, ortho derivative is 5 times less sensitive and meta derivative is a very weak substrate. When the benzoyl group bound on the N(alpha) arginine of BAPNA is replaced by a carbobenzoxy group (L-ZAPNA), the tryptic hydrolysis is increased by a factor of 3. With this latter substrate, the kinetic constants are determined for two forms, native and Nepsilon-acetylated of porcine and bovine trypsin. Evidence is presented that the rate of hydrolysis is both related to the global and local charges of the enzyme.