Cerebral metabolic rate of oxygen (CMRO₂) can be assessed quantitatively using ¹⁵O₂ and positron emission tomography. Determining the arterial input function is considered critical with regards to the separation of the metabolic product of ¹⁵O₂ (RW) from a measured whole blood. A mathematical formula based on physiologic model has been proposed to predict RW. This study was intended to verify the adequacy of that model and a simplified procedure applying that model for wide range of species and physiologic conditions. The formula consists of four parameters, including of a production rate of RW ( k) corresponding to the total body oxidative metabolism (BMRO₂). Experiments were performed on 6 monkeys, 3 pigs, 12 rats, and 231 clinical patients, among which the monkeys were studied at varied physiologic conditions. The formula reproduced the observed RW. Greater k values were observed in smaller animals, whereas other parameters did not differ amongst species. The simulation showed CMRO₂ sensitive only to k, but not to others, suggesting that validity of determination of only k from a single blood sample. Also, k was correlated with BMRO₂, suggesting that k can be determined from BMRO₂. The present model and simplified procedure can be used to assess CMRO₂ for a wide range of conditions and species.