Lymphangions, the segments of lymphatic vessels between valves, exhibit structural characteristics in common with both ventricles and arteries. Although once viewed as passive conduits like arteries, it has become well-established that lymphangions can actively pump lymph against an axial pressure gradient from low-pressure tissues to the great veins of the neck. A recently-reported mathematical model, based on fundamental principles, predicted that lymphangions can transition from pump to conduit behavior when outlet pressure falls below inlet pressure. In this case, the axial pressure gradient becomes the major source of energy for the propulsion of lymph, despite the presence of cyclical contraction. In fact, flow is augmented when cyclical contractions are abolished. We therefore utilized an in vitro preparation to confirm these findings and to test the hypothesis that lymphangion contraction inhibits flow when outlet pressure falls below inlet pressure. Bovine post-nodal mesenteric lymphatic vessels harvested from an abattoir were subjected to an inlet pressure of 5.0 cmH₂O and an outlet pressure that decreased from 6.5 to 3.5 cmH₂O under control conditions, stimulated with U46619 (a thromboxane analogue) and relaxed with calcium-free solution. Under control conditions, lymphatic flow increased markedly as outlet pressure fell below inlet pressure. In this case, the slopes of the flow versus axial pressure gradient increased with calcium-free conditions (61%, n=8, p=0.016) and decreased with U46619 stimulation (21%, n=5, p=0.033). Our findings indicate that stimulation of lymphatic contractility does indeed inhibit lymphatic flow when vessels act like a conduit.