One of the major difficulties in evaluating energy flow in natural animal populations is the determination of field metabolism. Various writers have suggested that there may be a correlation between excretion rate of a radioactive isotope and metabolism in small mammals which might be used to determine metabolism in free—ranging animals. To test this hypothesis, cotton rats confined in the laboratory and in field enclosures were injected with P32and their excretion rates were measured. Oxygen consumption also was measured in the laboratory animals. A poor correlation was found between oxygen consumption and excretion rate in the laboratory while a significant correlation was found between P32excretion rate, oxygen consumption and temperature. Assuming that this correlation held true in the field, oxygen consumption was predicted in the field population, as 2.37 cc O2/g hr—1 in July, 3.41 cc O2/g hr2o1in September and 4.02 cc O2/g hr—1in November. Oxygen consumption in the field was not significantly higher than in the laboratory, indicating that the cotton rats did not expend a significantly greater amount of energy in the field. The use of the excretion rate of a radioactive element as a predictor of metabolism was criticized due to the high variance and lack of correlation between excretion rate and metabolism at the individual level.