Cerebral metabolic rate for oxygen (CMRO(2)) and cerebral oxygen extraction fraction (OEF) are some of the most fundamental parameters to characterize the pathophysiologic status of cerebral tissue. Although O-15-labeled gases inhalation method is performed in clinical studies, application of the inhalation method on small animals requires too many intensive procedures. On this basis, the development of a new method to measure CMRO(2) and OEF in small animals is of interest. This study was aimed at developing a method to assess CMRO(2) and OEF using intravenously injectable oxygen (injectable 15O-O(2)) for small animals such as rats. Injectable 15O-O(2), 72 MBq/mL of radioactivity, was obtained after 15O-O(2) gas circulation into the artificial lung. OEF after injection of injectable 15O-O(2) was calculated using the same equation as that applied to the bolus inhalation of 15O-O(2) gas method. Values of 44 +/- 4.5 mL. min-1. 100 g-1 of CBF and 0.54 +/- 0.11 of OEF were obtained (n = 13). This OEF value was well accordance with OEF evaluated by arterial-venous difference of oxygen concentration (0.57 +/- 0.13). This method is useful to study the CMRO(2) and OEF in small animals using an animal positron emission tomography system. It may accelerate the basic research of several cerebral perfusion diseases.