Platelet integrins are proteins involved in the recognition of components from plasma and extracellular matrix. This recognition is essential for platelet adhesion and aggregation during haemostasis and arterial thrombosis. Disintegrins are cysteine-rich venom peptides (5-10 kDa) that inhibit integrin a _{IIbb 3}-dependent platelet aggregation. The interaction of integrins with disintegrins is dependent on a hairpin loop containing an Arg-Gly-Asp (RGD) motif maintained in an appropriate conformation by disulphide bridges. Herein we fractionated Bothrops jararaca venom isolating two known disintegrins, named jarastatin and jararacin, in order to better characterize their inhibitory properties towards human platelets aggregation. Furthermore, we compared these disintegrins with three distinct synthetic peptides for each disintegrin, based on the primary sequence of theses hairpin loop for jarastatin (CRRARGDDMDDYC, CRARGDDMDDC and CARGDDMDC) and for jararacin (CRRARGDNPDDRC, CRARGDNPDDC and CARGDNPDC). Using a combination of gel filtration, reverse-phase HPLC, strong cation exchange and confirming by MALDI-ToF mass spectrometry, we isolated jarastatin (7556 Da) and jararacin (7355 Da). These peptides showed potent inhibitory activity towards platelet aggregation following stimulation with ADP, with estimated IC₅₀ of 0.3 m M and IC₅₀ 75 nM, respectively. All cyclic peptides displayed lower activity even when tested at higher concentrations (100 m M). When platelets were stimulated by thrombin, the IC₅₀ for jarastatin was 120 nM and IC₅₀ for jararacin was 29 nM. In the same condition peptide CRRARGDDMDDYC from jarastatin showed an IC₅₀ of 66 m M and peptides CRARGDNPDDC and CARGDNPDC from jararacin showed IC₅₀ of 65 m M and 57 m M, respectively, showing that these cyclic peptides designed to mimicry the RGD-loop were not efficient to inhibit aggregation induced by ADP or thrombin. Thus, we concluded that other parts of these molecules are important for their activity. Moreover, the C-terminal of jarastatin and jararacin may have an important contribution in their interactions with platelet integrins, as already described for other disintegrins. Further investigations of both the structure and function of jarastatin and jararacin are needed to explore the inhibitory platelet aggregation properties of these disintegrins, possibly exploring their role on the pathophysiological alterations following envenomation by B. jararaca.