Estimates of the mean life span of the red blood cells of box turtles and South American giant toads were made with glycine-2-C14. The mean life span of the turtle red cells probably lies between 600 and 800 days and that of the toad between 700 and 1,400 days. The body weight, hematocrit, and hemoglobin values remained at relatively normal levels for 7 years in turtles and nearly 2 years in toads. These results indicate that the low metabolic rate, characteristic of poikilotherms, is correlated with a long red cell life span.

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