The mechanism of stimulation of platelets by thrombin and other proteases was studied by following kinetics of secretion of Ca2+ or ATP. The progress-time curves of secretion were analyzed for rate and total amount released. The reaction of thrombin was perturbed by addition of hydroxylamine or a competitive inhibitor and by variation of pH and it was compared with the reactions of other proteases. Trypsin and papain, with specificities for arginyl residues, induced secretion with a time course that was nearly identical with that induced by thrombin when saturating levels of enzyme were used. At low levels of enzyme, trypsin and papain gave extended lags in the progress-time curves. Higher concentrations of trypsin and papain were required for saturation of the measured parameters. Human plasmin (lysly specificity) and bovine chymotrypsin (aromatic amino acid specificity) failed to induce platelet secretion. Active site inhibited thrombin was also ineffective. Both yield and kinetics depended on pH, with the pH profile for each enzyme similar to its profile for hydrolysis of synthetic substrates. Studies at low pH also showed that the early part of the reaction undergoes a change in rate-determining step from enzyme dependent at low enzyme to enzyme indepdenent at high enzyme. Hydroxylamine, a nucleophile that would be expected to accelerate hydrolytic reactions, actually decreased both the rate of initial reactions and yield. A competitive inhibitor of thrombin also decreased both rate and yield; a calculated inhibition constant was in agreement with the value for a synthetic substrate, suggesting that the interaction of thrombin with platelets is analogous to reaction with substrates. A modification of our previous model is proposed in order to accommodate the results described here and to reaoncile the apparent contradictions that enzyme was found not to turn over in the reaction (Detwiler, T. C., and Feinman, R. D. (1973), Biochemistry 12, 282), that catalytic activity is required (Davey, M. G., and Luscher, E. F. (1967), Nature (London) 216, 875; this paper), and that the reaction is characterized by an apparent equilibrium binding (Tollefsen, D. M., Feagler J. R., and Majerus, P. W. (1974), J. Biol. Chem. 249, 2646). The essential feature is a reversible catalytic step with no dissociation of enzyme from product. This is followed by irreversible, thrombin-independent platelet processes leading to secretion, with yield dependent on the equilibrium concentration of the thrombin product. The model thus has aspects of catalysis, stoichiometry, and an agonist-receptor equilibrium.