A glucuronide derivative of exorphin C called exorphin C glucuronide was obtained by enzymatic synthesis and labeled with Tc-99m using glucoheptonate (GH) as bifunctional chelating agent. Labeling efficiency was 99.7 +/- 0.03%. Biodistribution studies were performed on normal and mammary tumor bearing albino Wistar rats. In addition, scintigraphic imaging with Tc-99m-GH-exorphin C glucuronide (Tc-99m-GEG) was performed. The radiopeptide-glucuronide is rapidly cleared via the urinary system showing lower liver uptake. Results demonstrated that Tc-99m-GEG can be used as a vehicle to target tumors.