Reported attempts to measure the flow field inside these vessels are few in number. (The term flow field refers to the flow velocity vector field in the mid-plane of a lymphatic vessel. A vector field is a collection of vectors of given direction and magnitude. Each vector is assigned to a given point in space.4) In fact, excluding studies on lymph filtration, the only study to quantify temporal variations of flow velocity during contraction was that conducted by Dixon and coworkers,5–7 which was subsequently used by Kassis et al.8 These studies were performed in situ, in the exteriorized rat mesenteric area. Lymphocytes were used as tracers and images were acquired with a high-speed camera. An image correlation algorithm was used to measure the lymphocyte velocity.9 The authors inferred the flow velocity, volumetric flow rate, and wall shear stress (WSS) by assuming that lymphocytes follow the flow faithfully, and that the flow is laminar and fully developed. A major drawback of this approach is that the lymphocytes are relatively large in size ( in diameter, whereas the vessel lumen diameter is, on average, ) and their density is usually low, reducing spatial resolution and, hence, the need to make assumptions with regard to the nature of the flow. Using lymphocytes as tracers limits the applicability to in situ measurements, and while such experiments may yield more physiologically relevant results with respect to an isolated in vitro preparation, the use of latter allows more control over factors that affect lymphatic function. Moreover, the latter study employed continuous wave (CW) illumination, which can significantly reduce the temporal resolution of an optical flow measurement system, rendering such a system less capable of resolving transient changes to flow velocity that may occur during the vessel contraction cycle.