Labeling of nitrilotris(methylene phosphonic)acid (NTMP) with rhenium-188 using stannous chloride as a reducing agent was investigated. Dependence of the yield of the 188Re-NTMP complex on the concentration of the reducing agent, pH, reaction time, temperature, ascorbic acid and amount of carrier added was studied. Under optimum conditions, the labeling yield of 188Re-NTMP complex was 90% for the carrier-free 188Re but with the carrier-added 188Re the labeling yield was more than 97%. Furthermore, the stability of the 188Re-NTMP complex against pH change and dilution with saline was also studied. It was found that the addition of carrier stabilized the 188Re-NTMP complex against pH change and dilution.