The llama fetus, a high altitude species has its basal peripheral vascular resistance (PVR), two times higher than fetuses of lowland species. AVP is an important regulator of the PVR in the latter. The aim of this study was to assess the AVP role in the maintenance of the femoral (FVR) and carotid (CVR) vascular resistance in normoxic conditions in the Ilama fetus. To that effect, we instrumented 5 fetal llamas at 0.59-0.72 of gestation, in which we continously determined the FVR and CVT before (45min) and after (15min) the administration of an VI receptor blocker d (CH2) 5-Try (Me) AVP (bolus of 240 ug×Kg plus infusion of 0.4ug × Kg × min). Four days before the experiment, we implanted ultrasonic flow transducers around the fetal femoral and carotid arteries and catheters were placed in femoral arteries and veins and in the amniotic cavities. The vascular resistance was calculated dividing the perfusion pressure by the blood flow. The antiAVP administration reduced the FVR from 2.36 ± 0.31 to 1.89 ± 0.22 mmHg × min/ml (× ± SEM; p<0.05), with no changes in CVR or arterial blood pressure. These results indicate that AVP plays a role in the maintenance of carcass vascular resistance in normoxia. However, the AVP role in producing the marked increased in PVR during hypoxia in the fetal Ilama, remains to be elucitated. (Fondecyt N° 197-0236 and Wellcome Trust).