Triatoma brasiliensis are vessel blood feeding arthropods and the main vector of Trypanossoma cruzi in the northeast of Brazil. They take very large blood meals, which can take up to 30 minutes to ingest. After blood ingestion, the anterior midgut secretes a protein that inhibits thrombin activity in T. infestans. This protein was named infestin. In order to study this anti-coagulant protein and its biological significance for the bug, the RNA interference emerges as a very useful technique. The aim of this study was to verify the effects of the RNA interference in the thrombin inhibitory activity of the T. brasiliensis midgut and evaluate the importance of the protein in the blood intake of insect. For such, the T. brasiliensis intestinal thrombin inhibitor was sequenced and primers were designed to synthesize double strand RNA (dsRNA). The protein was silenced by two injections of 15 mg of dsRNA into the thorax of fourth instar nymphs. Control groups were injected twice with saline alone or saline containing 15 mg of the ampicilin resistance gene (ARG) dsRNA. The guts were extracted 48 hours after the second injection and had the infestin mRNA expression verified by RT-PCR and activity verified by thrombin inhibitory activity assay (TIAA). The remaining bugs from each group were fed in hamsters. RT-PCR results showed that thrombin inhibitory transcripts were reduced by about 80% in silenced nymphs when compared to controls. The reduction in expression was confirmed by the TIAA using extracts of T. brasiliensis midgut. In control bugs, the amount of intestine necessary to inhibit 0.03U of thrombin was about 100 fold higher for the silenced group in comparison to the saline injected and ARG dsRNA injected group. Nymphs from the silenced group ingested significant lower amounts of blood (p<0.05, ANOVA). The results indicate that the anticoagulant activity in the midgut of T. brasiliensis, performed by the intestinal thrombin inhibitor, is important to determine the amount of blood taken from the host.