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The intense liver uptake of technetium-99-sestamibi (99mTc-MIBI) and photon scattering from the liver cause problems in quantitative perfusion interpretation. Hence, 99mTc-MIBI is a substrate for P-glycoprotein pump; variations in P-glycoprotein levels may affect liver clearance.Adult female Wistar rats were divided into seven main groups [control and St. John’s Wort (SJW) treated] and each SJW-treated group included three subgroups that were killed at 15, 30, and 45 min after 99mTc-MIBI injection. Treated groups received an SJW extract suspension at two doses of 100 and 400 mg/kg once daily for 5, 10, and 14 days, respectively. 99mTc-MIBI was injected intravenously to all rats 24 h after the final treatment. The rats were anesthetized at the mentioned time after tracer injection, and heart and liver tissues were removed, weighed, and their radioactivity was counted. One rat from each group was selected randomly for myocardial perfusion imaging. A significant increase in liver clearance and heart-to-liver ratio was observed in all SJW-treated groups compared with the control, especially at 10 days after SJW treatment. The heart radioactivity decreased in SJW-receiving groups at 14 days after SJW treatment.This study showed that SJW extract accelerates the liver clearance of 99mTc-MIBI and significantly reduces photon scattering from the liver.