Lymph formation and progression in submesothelial lymphatics depends upon transmural pressure gradients developing between the interstitial space and the lymphatic lumen or between adjacent lymphatic segments. This is a very complex and not completely unveiled mechanism; therefore, the aim of the present project was to study the kinetics of fluid flux to clarify the function of the organization of the lymphatic network. Experiments were performed on long linear vessels and complex loops belonging to the submesothelial lymphatic diaphragmatic network of anesthetized, paralysed and mechanical ventilated rats. In a single vessel, both lymphatic intraluminar hydraulic pressure (P_lymph) and lymph flow were recorded by means of the micropuncture technique combined with the injection of fluorescent dextrans to visualise lymph flow. Direct measurements of the fluorescent front progression and vessels diameter were performed on acquired images to calculate lymph flow (F_L) and flow velocity (v_L). Data show that F_L, v_L and flow direction vary considerably with time even in the same vessel (range: from 1 ± 0.3 (1 SE) nL/min to 154.9 ±10.4 nL/min). In addition, in some vessels, flow direction reverts at a frequency (about 11.3 ± 1.6 Hz) not related to the cardiac or respiratory rates. The fact that flow direction was independent upon the intraluminar P_lymph pressure gradient, suggests the existence of an active pumping mechanism in these vessels.