Psidium guajava (guava) leaf is a phytotherapic used in folk medicine to treat gastrointestinal and respiratory dis-turbances and is used as anti-inflammatory medicine. In nuclear medicine, blood constituents (BC) are labelled with techne-tium-99m (99mTc) and used to image procedures. However, data have demonstrated that synthetic or natural drugs could modify the labelling of BC with 99mTc. The aim of this work was to evaluate the effects of aqueous extract of guava leaves on the labelling of BC with 99mTc. Blood samples of Wistar rats were incubated with different concentrations of guava extract and labelled with 99mTc after the percentage of incorporated radioactivity (%ATI) in BC was determined. The results suggest that aqueous guava extract could present antioxidant action and/or alters the membrane structures involved in ion transport into cells, thus decreasing the radiolabelling of BC with 99mTc. The data showed significant (P<0.05) alteration of ATI in BC from blood incubated with guava extract. Psidium guajava (guava) leaf is a phytotherapic used in folk medicine to treat gastrointestinal and respiratory dis-turbances and is used as anti-inflammatory medicine. In nuclear medicine, blood constituents (BC) are labelled with techne-tium-99m (99mTc) And used to image procedures. However, data have demonstrated that synthetic or natural drugs could modify the labelling of BC with 99mTc. The aim of this work was to evaluate the effects of aqueous extract of guava leaves on the labelling of BC with 99mTc. Blood. The samples of Wistar rats were incubated with different concentrations of guava extract and labelled with 99mTc after the percentage of incorporated radioactivity (%ATI) in BC was determined. The results suggest that aqueous guava extract could have antioxidant act and/or alters the In ion transport into cells, therefore decreasing the radiolabelling of BC with 99mTc. The data showed significant (P<0.05) alteration of ATI in BC from blood culture With guava extract.