Ginkgo biloba is the phytoterapic most used in popular medicine in the treatment of cerebral senescence. Red blood cells (RBC) labeled with technetium-99m (Tc-99m) is used for several evaluations in nuclear medicine. This labeling depends on a reducing agent, usually the stannous ion. Any drug, which alters the labeling of the tracer, could be expected to modify the disposition of the radiopharmaceutical. We have evaluated the influence of the Ginkgo biloba extract on the labeling of RBC and plasma proteins with Tc-99m. Blood was withdrawn and incubated with Ginkgo biloba extract (0; 0.004; 0.04; 0.4; 4; 20 and 40 mg/ml). Stannous chloride (1.2 ml/ml) was added and, then, Tc-99m was added. Plasma (P) and blood cells (RBC) were isolated, also precipitated with trichloroacetic acid and soluble (SF) and insoluble fractions (IF) separated. The analysis of the results shows that there is a decrease in the radioactivity (from 97.7 ± 0.7 to 49.5 ± 3.9%) in RBC with the drug (4 mg/ml). In the labeling process of RBC with Tc-99m, the stannous and pertechnetate ions pass though the membrane, so, we suggest that the Ginkgo biloba effect can be explained by (i) an inhibition of the transport of these ions, (ii) damage in membrane, (iii) competition with the cited ions for the same binding sites, or (iv) possible generation of reactive oxygen species that could oxidize the stannous ion.