Objectives. The present study was initiated to explore the effects of hydroxyethylidene diphosphonate labeled with rhenium 186 (186Re-HEDP) treatment on the progression of lumbar skeletal metastasis in an animal model and to correlate the eventual treatment efficacy with the radioisotope tissue distribution. Methods. The effect of 186Re-HEDP on the progression of lumbar metastasis from prostate cancer was investigated in the Copenhagen rat model. Metastatic prostate tumor deposits were induced in male rats by tail vein injection of R3327-MATLyLu prostate tumor cells under concomitant clamping of the inferior caval vein. The development of clinical symptoms such as onset of hind leg paralysis and urinary bladder swelling was monitored and related to the presence of tumor cells within histologic sections of L-5 and L-6 vertebrae. Results. The 186Re-HEDP administration, given either 1 day or 8 days after surgical induction of lumbar metastasis, could significantly increase the symptom-free survival of the animals. These results were confirmed by a significant decrease in the presence of histologically detectable tumor tissue. Biodistribution studies demonstrated the uptake of the major part of the radioisotope within bone tissue. Uptake of radioactivity within the lumbar vertebrae on a microscopic scale, as shown by phosphor screen autoradiography, was concentrated in areas of bone formation and turnover. Signs of radiotoxicity, such as bone marrow replacement by fat cells and the absence of megakaryocytes, were observed. Conclusions. The results show that radionuclide treatment using 186Re- HEDP is a potentially efficacious treatment option in prostate cancer disseminated to the skeleton. The optimal treatment dose should be determined carefully and aimed at acceptable levels of myelotoxicity.