Faber J, Anderson D, Hohimer R, Yang Q, Giraud G, and Davis L. The function curve of the membranes that regulate amniotic fluid volume in sheep. Am J Physiol Heart Circ Physiol XXX: HXXX-HXXX, XXXX. - Seven singleton 120 day fetal lambs were prepared with a shunt from the lung to the gastric end of the esophagus, a bladder catheter and multiple amniotic fluid catheters and vascular catheters. The urachus was ligated. Beginning 7 days later, amniotic fluid volumes were determined by drainage, followed by replacement with 1 liter lactated Ringer's solution (R). Urine flow into the amnion was measured continuously. In 14 of 27 experiments, amniotic fluid volumes were determined again 2 days after the inflow into the amnion had consisted of urine only and in 13 experiments after the inflow of urine had been supplemented by an intraamniotic infusion of R. Intramembranous absorption was calculated from the inflows and the changes in volume between the beginning and the end of each experiment. The relations between absorption rate and amniotic fluid volume, the function curves, were highly individual. Urine production during infusion of R did not decrease; fetal PRA decreased (P<0.001); amniotic fluid volume increased by 140 % (SEM 27%, P<0.005) but the increase in the amniochorionic absorption rate of 411% (SEM 48%, P<0.001) was greater (P<0.005) than the increase in volume. Each of the 7 fetuses was proven capable of an average intramembranous absorption rate that exceeded 4.5 liters of amniotic fluid per day. During infusion of R, the increase in rate of absorption matched the rate of infusion (both in ml/hr), with a regression coefficient of 0.8 (P<0.01). We conclude that, even for large amniotic fluid volumes, volume is not limited by the absorptive capacity of the amniochorion and that, in these preparations, the position of the function curve and not the natural rate of inflow was the major determinant of resting amniotic fluid volume.