Introduction: The use of ^99mTc-macroggregated albumin for lung perfusion imaging is well established in nuclear medicine. However, there have been safety concerns over the use of blood-derived products because of potential contamination by infective agents, for example, Variant Creutzfeldt Jakob Disease. Preliminary work has indicated that Tc(CO) ₅I is primarily taken up in the lungs following intravenous administration. The aim of this study was to evaluate the biodistribution and pharmacokinetics of ^99mTc(CO) ₅I and its potential as a lung perfusion agent. Methods: ^99mTc(CO) ₅I was synthesized by carbonylation of ^99mTcO _4- at 160 atm of CO at 170°C in the presence of HI for 40 min. Radiochemical purity was determined by HPLC using ⁹⁹Tc(CO) ₅I as a reference. ^99mTc(CO) ₅I was administered by ear-vein injection to three chinchilla rabbits, and dynamic images were acquired using a gamma camera (Siemens E-cam) over 20 min. Imaging studies were also performed with ^99mTc-labeled macroaggregated albumin ( ^99mTc-MAA) and ^99mTcO _4- for comparison. ^99mTc(CO) ₅I was administered intravenously to Sprague-Dawley rats, and tissue distribution studies were obtained at 15 min and 1 h postinjection. Comparative studies were performed using ^99mTc-MAA. Results: Radiochemical purity, assessed by HPLC, was 98%. The retention time was similar to that of ⁹⁹Tc(CO) ₅I. The dynamic images showed that 70% of ^99mTc(CO) ₅I appeared promptly in the lungs and remained constant for at least 20 min. In contrast, ^99mTcO _4- rapidly washed out of the lungs after administration. As expected ^99mTc-MAA showed 90% lung accumulation. The percentage of injected dose per gram of organ ±S.D. at 1 h for ^99mTc(CO) ₅I was as follows: blood, 0.22±0.02; lung, 12.8±2.87; liver, 0.8±0.15; heart, 0.15±0.01; kidney, 0.47±0.08. The percentage of injected dose per organ ±S.D. at 1 h was as follows: lung, 22.47±2.31; liver, 10.53±1.8; heart, 0.18±0.01; kidney, 1.2±0.17. Tissue distribution studies with ^99mTc-MAA showed 100% lung uptake. Conclusion: ^99mTc(CO) ₅I was synthesized with a high radiochemical purity and showed a high accumulation in the lungs. Further work on the mechanism and optimization of lung uptake of ^99mTc-pentacarbonyl complexes is warranted.