Fluid and solute flux between the pleural and peritoneal cavities, although never documented under physiological conditions, might play a relevant role in pathological conditions associated to the development of ascitis and pleural effusion and/or in the processes of tumour dissemination. To verify whether a pleuro-peritoneal flux might take place through the diaphragmatic lymphatic network, the transdiaphragmatic pressure gradient (P_TD) was measured in 5 spontaneous breathing anesthetized rats. P_TD was -1.93 ± 0.59 cmH₂O (1 SD) and - 3.1 ± 0.82 cmH₂O at end-expiration and at end-inspiration, respectively, indicating the existence of a pressure gradient directed from the abdominal to the pleural cavity. Morphometrical analysis of the diaphragmatic lymphatic network was performed in the excised diaphragm of 3 additional rats euthanized with an anaesthesia overdose. Optic and electron microscopy revealed that lymphatic submesothelial lacunae and lymphatic capillaries among the skeletal muscles fibres show the ultrastructural features of the so-called initial lymphatic vessels, namely a discontinuous basal lamina and anchoring filaments linking the outer surface of the endothelial cells to connective tissue or to muscle fibres. Primary unidirectional valves in the wall of the initial lymphatics allow entrance of serosal fluid into the lymphatic network preventing fluid backflow, while unidirectional intraluminar valves in the transverse vessels convey lymph centripetally towards central collecting ducts. The complexity and anatomical arrangement of the two valves system suggests that, in spite of the existence of a favourable P_TD, in physiological condition no fluid bulk flow takes place between the pleural and peritoneal cavity through the diaphragmatic lymphatic network.