We investigated the in vitro effects of the site-directed thrombin inhibitor-a single-stranded oligonucleotide aptamer (GGTTGGTGTGGTTGG)-on thrombin proteolytic activity towards its two natural substrates: fibrinogen and platelet thrombin receptor (PAR-1). The thrombin aptamer was shown to strongly affect thrombin clotting activity at nanomolar concentrations and thrombin-dependent degradation of proteolytically activatable receptor, PAR-1, exposed on platelet surface membrane at micromolar concentrations. The incubation of PPP with thrombin in the presence of 100-1000 nM aptamer resulted in the significant concentration-dependent prolongation of thrombin time (up to fourfold, P<.0001). Aptamer significantly reduced the thrombin-induced platelet degranulation (46+/-20% inhibition at 0.15 U/ml thrombin, P<.001), as well as thrombin-mediated platelet aggregation in PRP (7+/-10% inhibition at 1 U/ml thrombin, P<.05). Furthermore, aptamer inhibited the thrombin-catalysed cleavage of PAR-1 in a dose-dependent manner, i.e., by 17%, 27% and 70%, respectively, for the concentrations of 100, 500 and 1000 nM (P<.025 by randomised block analysis; P(regression slope)<.0001). We conclude that aptamer is able to considerably attenuate thrombin proteolytic activity regardless of the molecular size of thrombin substrates. Our observations directly proved that aptamer may be successfully used for the inhibition of thrombin activity towards various physiological targets: one related to fibrin generation in the final stage of coagulation cascade, and another concerning the interaction of thrombin with its surface membrane receptor, PAR-1, in blood platelets.