Single and double indicator-dilution measurements of the circulating blood volume were made in chronically catheterized sheep fetuses averaging 131 days gestation. Blood volume measured with 51Cr-labeled red blood cells averaged 110.3 +/- 10.7 (SD) ml/kg with a range of +/- 15% of the mean (n = 23). The 125I-albumin (n = 7) and 125I-fibrinogen (n = 5) blood volumes averaged 126.2 +/- 9.8 (SD) and 124.5 +/- 6.9 ml/kg, respectively. The double-indicator fetal blood volume averaged 120.6 +/- 8.3 ml/kg (n = 12); however, this value is too high due to the assumptions used when determining plasma volumes. In addition, after either a 15% fetal hemorrhage or intravenous epinephrine infusion, there was no statistically significant release of red blood cells into the circulation. Labeled maternal red blood cells were removed from the fetal circulation at an average rate of 18%/h, whereas there was no detectable loss of labeled autologous fetal cells from the circulation. Finally, there were spontaneous changes in fetal blood volume including an average decrease of 2.3 +/- 0.3% (SE) (n = 10) during low-level uterine contractions (i.e., contractures). In summary, it appears that labeled fetal red blood cells provide the most accurate estimate of circulating fetal blood volume because of unresolvable problems in extrapolating the plasma label back to zero time. In addition, changes in blood volume can be accurately calculated from either hematocrit or hemoglobin concentration because there appeared to be no red cell reservoirs in the fetuses.