Tlie "equilibration" method introduced by Bradley to measure circulating "splanchnic blood volume" has been applied to the estimation of "pulmonary blood volume." The theoretic and practical aspects of the method are analyzed. Values for circulating "pulmonary blood volume" determined in anesthetized dogs were not significantly different from values obtained by the "mean circulation time output" method and were significantly greater than those determined by the "slope output" method. When experimental conditions were varied, relatively parallel changes were recorded by the three methods. It is believed that the "equilibration" method is a valid one for estimating circulating "pulmonary blood volume."