The feto-placental blood volume was studied using a hemoglobin hemodilution technique in 15 erythroblastotic fetuses at 43 intravascular transfusions. Four severely anemic fetuses (Hb 30 g/l) had significantly larger blood volumes than 11 moderately anemic fetuses (Hb 30-79 g/l), 169 +/- 37 versus 105 +/- 32 ml/kg fetal body weight (p less than 0.001). We speculate that hypovolemia might be an adaptive change to maintain an adequate hemoglobin concentration. An escape of fluid from the intra- to the extra-vascular space will probably compensate for the reduction in total red cell mass and thus slightly increase the hemoglobin concentration. Theoretically, interstitial fluid accumulation will continue until the hydrostatic pressure of the extravascular tissue balances that of the capillary. This compensatory mechanism seems to function until the hemoglobin concentration drops below, 30 g/l, at which point the blood volume will increase, suggesting a change from a hypo- to a hyper-volemic state.