SummaryExperiments were performed on 11 long-term fetal lamb preparations (103–138 days of gestation) to investigate the sensitivity and relative responsiveness of the fetal volume receptors in modulating fetal plasma arginine vasopressin (pAVP) secretion and plasma renin activity (PRA) secretion during fetal hypovolemia and after fetal blood volume replacement. During fetal hemorrhage there were significant decreases (P< 0.05) in fetal hematocrit (34.7 ± 2.58 to 27.0 ± 1.64%), plasma proteins (3.14 ± 0.15 to 2.78 ± 0.19 g/100 ml), mean arterial blood pressure (MABP) (58.1 ± 2.59 to 52.2 ± 2.60 mmHg) and fetal arterial pH (7.38 ± 0.01 to 7.35 ± 0.01). A significant increase in fetal pAVP concentration from 0.73 ± 0.21 to 34.9 ± 10.04 μU/ml (P< 0.01) and fetal PRA from 4.78 ± 2.22 to 40.4 ± 18.31 ng/ml/hr (P< 0.05) was demonstrated at the peak of fetal hemorrhage. Two hr after correction of the fetoplacental blood volume, these values were back to base line levels. No change in either maternal pAVP or PRA was seen during fetal hemorrhage. When individual values for log pAVP and log PRA were plotted as a function of percent of fetoplacental blood volume removed the correlation coefficients were 0.82 and 0.60, respectively. A multiple regression analysis showed a high correlation of log pAVP and log PRA with the volume of blood removed and a low partial correlation with the fetal MABP. This suggests the decrease in fetal MABP was not the primary factor explaining the increase in pAVP and PRA during fetoplacental blood volume depletion. The data indicate that the fetal volume receptors for control of arginine vasopressin secretion are fully functional in the last trimester of gestation and suggests that fetal pAVP and PRA are released as an exponential function of the percent of fetoplacental blood volume depletion. Finally, an isosmotic water shift from the fetal interstitial space to the fetal vascular space is described during fetal hemorrhage.SpeculationIt is suggested that a change in the equilibrium between the forces regulating fluid movement through the fetal capillary membranes, in accordance with Starling's principle, activates isosmotic water fluxes from the fetal interstitial space to the fetal vascular compartment counteracting the effects of fetal blood volume depletion. Therefore, the role of arginine vasopressin (AVP) release during fetal hemorrhage, if any, will be to act as a pressor substance helping to maintain fetal blood pressure. No major effect of AVP on placental membranes was demonstratedin vivo.