The goal of this study was to determine the radiopharmaceutical potential of radioiodinated thebaine. Thebaine was extracted from dry capsules of opium poppy (Papaver somniferum L.), purified using high-performance liquid chromatography, and characterized with nuclear magnetic resonance and infrared spectroscopy. The purified thebaine was labeled with I-131 using the iodogen method. Normal and receptor-blockage biodistribution studies were performed in male Albino Wistar rats. The results of the tissue distribution studies showed that the uptake of I-131-thebaine in the stomach, large intestine, spinal cord, and prostate was higher than in the other tissues. A greater uptake of radiolabeled thebaine in the rat brain was observed in the midbrain and hypothalamus. We concluded that (1) the labeling yield of I-131-thebaine was high, (2) a large amount of I-131-thebaine remained in the midbrain and hypothalamus, and (3) I-131-thebaine had enough stability for diagnostic scanning.