The protease-activated receptor (PAR) belongs to the large superfamily of G-protein-coupled seven trans-membrane domain receptors. The activation of PARs is achieved by proteolytic unmasking of the cryptic N-terminal receptor-activating sequence that binds to the body of the same receptor molecule. PARs-1,-3 and-4 are activated by thrombin, while PAR-2 is activated by trypsin or mast cell tryptase, but not by thrombin. PARs are weidely distributed to a variety of tissues and participate in a number of physiological or pathophysiological phenomena such as platelet aggregation, inflammation and cardiovascular, digestive or respiratory functions. Thus, PARs are of physiological importance and also of pharmacological interest as the novel target for drug development.