OBJECTIVE: Our purpose was to determine whether the increase in extravascular fluid in chronic fetal anemia occurs either because of heart failure or despite successful cardiac adaptation. STUDY DESIGN: Right ventricular function curves were obtained in five ovine fetuses at the start, midpoint, and end of 5 to 8 days of anemia induced by isovolemic daily hemorrhage. Least-squares fit of the ascending and plateau lines of stroke volume versus right atrial pressure were used to establish breakpoints (intersection of the ascending and plateau lines), which were compared by analysis of variance for repeated measures. Myocardial blood flow was measured by microspheres. RESULTS: Carotid arterial oxygen content was reduced from 7.0 ± 0.3 to 2.1 ± 0.1 ml/dl and the hematocrit from 29% ± 1.8% to 13% ± 0.6%. Breakpoint analysis of function curves showed that although right atrial pressure remained unchanged (3.4 ± 0.7 and 3.6 ± 0.6 mm Hg) stroke volume increased from 1.03 ± 0.14 to 1.62 ± 0.25 ml/kg. Both right and left ventricular coronary blood flow were increased, 1351 ± 313 and 1166 ± 264 ml/min per 100 gm. Excess fluid was present in the abdomen and chest of most animals at autopsy. CONCLUSION: Tissue edema during severe anemia occurs despite normal right atrial pressure, increased stroke volume, and markedly increased coronary blood flow, markers of successful cardiac adaptation. (AM J OBSTET GYNECOL 1996;174:1289-94.)