Technetium-99m exametazime (99mTc-d,l-HMPAO), prepared by the reconstitution of the Ceretec kit, is widely used in the investigation of regional cerebral blood flow. The radiochemical purity specification of not less than 80% lipophilic complex requires that the kit is used within 30 min of reconstitution. In certain circumstances this imposes restrictions on its clinical availability. A number of approaches to extending the 30-min shelf life have been proposed and these are discussed. A method of stabilising the kit by the addition of 200 micrograms cobalt chloride hexahydrate (CoCl2 x 6H2O) in 2 ml of water is described. The addition of this solution can extend the shelf life of the reconstituted kit to at least 5 h post reconstitution.