Fetal ventricular performance has been considered limited because ventricular output does not increase with rapid volume expansion above mean left atrial pressures (mLAPs) of 5–7 mm Hg. To explore relations between preload, afterload, and stroke volume (SV) in the fetal left ventricle, we instrumented 126–129 days gestation fetal lambs with ascending aortic electromagnetic flow transducers, vascular catheters, and inflatable occluders around the aortic isthmus (n=8) or descending aorta (n=7). At 24–48 hours after surgery, blood was withdrawn or infused to reach various mLAPs. The aorta was then slowly occluded as aortic flow and mean arterial pressure (MAP) were measured continuously. Isthmus constriction produced linear decreases in SV as MAP increased; mLAP was unchanged. Descending aortic constriction produced no decrease in SV until high MAPs were reached. SV decreased as MAP increased further, and mLAP rose significantly. The curve relating mLAP and SV before constriction showed little increase in SV above mLAPs of 5–7 mm Hg; however, when curves were derived relating SV and mLAP at relatively constant MAPs, SV continued to increase even above an mLAP of 8–10 mm Hg. Our studies indicate that the fetal left ventricle responds to progressive increases in mLAP to at least 10 mm Hg. The lack of increase in SV above an mLAP of 5–7 mm Hg with rapid volume expansion is related to the concomitant increase in MAP and afterload. {Circulation Research 1989;65:127–134)