Technetium-99 m-hexamethylpropyleneamine oxime (99mTc- HMPAO) is a neutral-lipophilic chelate which is used for scanning cerebral blood flow. The labeling efftciencies of 99mTc-HM-PAO is known to be sensitive to the amount of pertechnetate added and the quality of the pertechnetate. Because of these factors, the manufacture recornmends that HM-PAO kits be reconstituted with a maximum of 30 mCi pertee.hnetate which was eluted <4 hr earlier from a generator which had been eluted <24 hr previously. So we measured the labelling efficiencies and the decomposition rate constant according to the amount of pertechnetate added, the volume of pertechnette added, and generator in-growth time. We used the 3-system chromatagraphic rnethods (paper & ITLC-SG chrornatography) which analyzed the labelling efficiencies of the 99mTc-HM- PAO. There was no significant difference in labelling efficiencies between variable pertechnetate acitvities added. (39.9 +- 4.9mCi: 87.8 +- 5.1 (%), 60.8 +- 5.0mCi: 90.7 +- 2.2(%), 79.0 +- 6.0mCi: 86.8 +- 3.9 (%), 106.6 +- 11.6mCi: 87.7 +- 1.2 (%), p>0.05) No significant difference in labelling efficiencies were found between pertechnetate of 4 ml and 5 ml. (4 ml: 89.1+3.2 (%), 5 ml: 87.3+4.0 (%), p> 0.05) There was no difference between 1-6 and 10 +-48 hr of generator in- growth time. (1-6 hr: 87 8+ 4.0 (%), 10 +- 48 hr: 89.6+1.6 (%), p>0.05) The mean value of decompositian rate constant was 0.196+ 0.097 (hr-1), and there were no differerce according to the amount of pertecnetate added and the volume of pertecnetate added, (39.9+4.9 mCi: 0.208+0.059 (hr-1), 60.8 +- 5.0 mCi: 0.191 +- 0.100 (hr-1) 79.06.0mCi: 0.192 +- 0.118 (hr-1 ), 106.6 +- 11.6mCi: 0.212 +- 0.030 (hr-1), p>0.05, 4ml: 0.200+0.074 (hr-1), 5mi: 0.193 +- 0.115 (hr-1), p>0.05) In the case of using the first eluate, the labeliing efficiency of 99mTc-HM-PAO was 82.1%.. These data suggest that there were no significant aiteration in labelling efficiency of 99m Tc-HM- PAO aecording to the considerable range of pertechnetate activities and volume added, and generator in-growth time. Also, it was shown that one vial of HM-PAO kit supplied the Tc-HM-PAO which was used for 3-4 patients.