SummaryEndogenous bile pigment excretory rates have now been recorded in fifteen species, and the following review briefly compares the excretion per unit of body weight in animals of varying sizes, attempts to determine if any correlation exists between bile flow and endogenous pigment excretion and discusses bile pigment excretion in relation to mean erythrocyte survival time and metabolic rate. Graphs and one table illustrating pigment metabolism and erythrocyte replacement are presented. Differences between animals excreting primarily biliverdin as opposed to bilirubin are discussed, with special attention being given throughout the article to mammals and their individual differences. Accepted theory concludes that the rate of endogenous bile pigment excretion is primarily dependent upon hemoglobin cata‐bolism. However, further studies will be necessary to clarify basic physiological mechanisms in order to substantiate any theory concerning the correlation of erythrocyte turnover with metabolic rate