T he formation of bilirubin-¹⁴C was measured in rats given transfusions of red blood cells containing ¹⁴C-labeled hemoglobin heme. Per cent conversion of hemoglobin-¹⁴C to bilirubin was 4 times greater with transfusion of “stress” reticulocytes from rats responding to hemorrhage than with normal reticulocytes from unstimulated donors. When the increased number of labeled reticulocytes produced by hemorrhaged donors was also considered, the total magnitude of labeled bilirubin formation was almost 20 times higher with stress as compared to normal reticulocytes. The findings were not influenced by splenectomy of either donor or recipient rats, iron loading of donors, or bleeding of recipients. However, bilirubin-¹⁴C formation fell off progressively as studies were performed at longer intervals after erythroid stimulation.Total bilirubin-¹⁴C formation in rats transfused with stress reticulocytes was compared to the production of early-labeled bilirubin from all potential sources in intact rats bled according to the same schedule used in the transfusion experiments. It is estimated that degradation of hemoglobin from sress reticulocytes accounts for virtually the entire rise in erythropoietic bilirubin formation from 24 to 96 hr after glycine-2-¹⁴C administration, but that additional sources make a major contribution before that time. These findings are consistent with the concept that destruction of immature erythroid cells in the peripheral blood, and probably in the bone marrow, accompanies the physiologic response to erythroid stimulation.