Bone marrow smears and blood samples were examined in guinea pig fetuses in which intrauterine growth retardation (IUGR) had been induced by uterine artery ligation and compared with those of control (well-grown) fetuses from uterine horns with intact circulation. Differential bone marrow cell counts were obtained from a count of 300 cells per smear and blood samples were assayed for hemoglobin concentration and 2,3-diphosphoglycerate (DPG). Results of blood assays showed no difference in hemoglobin concentration. DPG levels were reduced in the IUGR guinea pigs (P < .05), which could be a consequence of decreased glucose availability or represent an adaptation to reduced oxygen availability. Comparisons of bone marrow counts revealed an increase in total erythrocyte precursors (P < .05) and a decrease in total granulocytic precursors (P < .05) in IUGR fetuses. Within the erythroid lineage there was a significant increase in late (orthochromatic) erythroblasts (P < .005) in the IUGR animals compared with control animals. The granulocytic lineage of the IUGR fetuses showed a significant decrease in mature neutrophils (P < .05) and eosinophilic precursors (P < .05) compared with controls. These data suggest that the hypoxic stress of uterine artery ligation leads to an increase in medullary erythropoiesis. In concert with a previous study that showed a reduction in hepatic erythropoiesis, these data suggest a precocious shift of the anatomic site of erythropoiesis from the liver to the bone marrow under conditions of hypoxia.