The cerebral metabolic rate for oxygen (CMRO2) and cerebral oxygen extraction fraction (OEF) are two fundamental parameters used to characterize the pathophysiologic status of cerebral tissues. Although the O-15-labeled gas inhalation method is used to measure these parameters in clinical studies, applying this method to small animals requires many intensive procedures. Thus the development of a new method to measure CMRO2 and OEF in small animals is of interest. This study aimed to develop a method to assess CMRO2 and OEF using intravenously injectable oxygen (injectable 15O-O2) and Positron Emission Tomography (PET) for small animals such as rats. Injectable 15O-O2 was obtained after 15O-O2 gas circulation into an artificial lung. The OEF in normal rats was calculated using the same equation as that used for the bolus inhalation in the 15O-O2 gas method. The obtained value of 0.54±0.11 of OEF was similar to the value determined from the arterial-venous difference in the oxygen concentration. Furthermore, we evaluated the usefulness of the injectable 15O-O2 system in rats occluded in the right middle cerebral artery. A decrease in cerebral blood flow (CBF) and compensatory increase in OEF were observed 1 h after occlusion. In contrast, a marked decrease in CBF and CMRO2 and a collapse of the compensatory OEF mechanism were found 24 h after occlusion. Thus injectable 15O-O2 with PET can be used to estimate oxygen metabolism reliably in stroke animal models, and may be useful for accelerating basic research on cerebral diseases.