The trabecular meshwork (TM) and Schlemm's canal (SC) are responsible for generating the majority of outflow resistance, however the distal regions of the conventional outflow pathway appear to account for 25-50% of total. Sections of these distal vessels are surrounded by α-smooth muscle actin containing cells, indicating that they may be vasoregulated. This study examined the effect of a potent vasodilator, nitric oxide (NO) and its physiological antagonist endothelin-1 (ET-1) on the regulation of outflow resistance in the distal regions of the conventional outflow pathway. Using a physiological model of the conventional outflow pathway, human and porcine anterior segments were perfused in organ culture under constant flow conditions, while intrachamber pressure was continually monitored. For porcine anterior segments, a stable baseline outflow facility with TM intact was first achieved before anterior segments were removed and a trabeculotomy performed. For human anterior segments, a trabeculotomy was immediately performed. In human anterior segments, 100 nM ET-1 significantly decreased distal outflow facility from 0.49{plus minus}0.26 to 0.31{plus minus}0.18 (mean{plus minus}SD) µl/min/mmHg, p<0.01, a decrease of 38{plus minus}16%. Perfusion with 100µM DETA-NO in the presence of 1 nM ET-1 immediately reversed ET-1 effects, significantly increasing distal outflow facility to 0.54{plus minus}0.35 µl/min/mmHg, p=0.01, an escalation of 175{plus minus}49%. Similar results were obtained in porcine anterior segment experiments. In conclusion, data show a dynamic range of resistance generation by distal vessels in both the human and porcine conventional outflow pathways. Interestingly, maximal contraction of vessels in the distal outflow tract generated resistance very near physiological levels for both species.