We investigated the relation of the age trajectory of physiological indicators of the average metabolic activity of organisms in a population to the age-specific population mortality rate. We show that a metabolic rate indicator (MRI) can be estimated using traditional physiological measures, such as homeostatic serum glucose concentration, vital capacity, and such. Estimates of the MRI were made from data collected in the Multiple Risk Factor Intervention Trial (MRFIT) study. The relation of the empirical mortality rate and MRI was also tested using MRFIT data. The age trajectory of MRI was evaluated using Swedish mortality data. The mortality results reproduce the "Strehler and Mildvan effect." The average rate of decline of MRI with age coincides with estimates predicted by Strehler using other methods. Possible extensions of the method are discussed.