The tripeptide aldehyde GYKI-14766 (D-MePhe-Pro-Arg-H) synthesized by Bajusz et al. in 1975 is a specific, reversible thrombin inhibitor. It was found effective in vitro in clotting time assays as well as in vivo in thrombosis models. To study the biochemical effects of the inhibitor various experimental setups were applied. First we measured the binding of thrombin to platelets using 125J-thrombin. KD was 55 nM. Second, 125J-thrombin was displaced by thrombin or by a GYKI-14766-thrombin-complex with similar efficacy. However, the binding of thrombin to the platelets increased the intracellular free Ca2+ concentration, but the inhibitor-thrombin complex did not influence it. Analyzing the kinetics of the reactions involved we found that the formation of the GYKI-14766-thrombin complex was slower than the triggering of the platelet Ca2+ signal by thrombin.