WHEN TECHNETIUM TC99M PERTECHNETATE IS ADDED TO EXAMETAZIME IN THE PRESENCE OF STANNOUS REDUCTANT, A LIPOPHILIC TECHNETIUM TC99M COMPLEX IS FORMED. THIS LIPOPHILIC COMPLEX IS THE ACTIVE MOIETY. IT CONVERTS AT APPROXIMATELY 12%/HOUR TO LESS LIPOPHILIC SPECIES. WHEN THE SECONDARY COMPLEX IS SEPARATED FROM THE LIPOPHILIC SPECIES, IT IS UNABLE TO CROSS THE BLOOD-BRAIN-BARRIER. THE USEFUL LIFE OF THE RECONSTITUTED AGENT IS LIMITED TO 30 MINUTES. THE IN VITRO ADDITION OF METHYLENE BLUE TO THE TC99M-EXAMETAZIME WILL STABILIZE THE COMPLEX FOR 4-6 HOURS. METHYLENE BLUE MAY BE ADDED TO TC99M FOR CEREBRAL IMAGING. METHYLENE BLUE SHOULD NOT BE USED IN THE PREPARATION OF TC99M-EXAMETAZIME LABELED LEUKOCYTES.