In serial sections from the oedematous skin of hind limb of rats the lymph vessel walls were studied for structures active in propelling the lymph. To dilate the lymphatics lymph formation and transport were increased by dextran-oedema. A special type of sack-like structure lined almost exclusively by endothelial cells was observed at the postcapillary segment between the lymph capillaries and lymph vessels proper. The lymph flow from the capillaries into these postcapillary sack-like sinuses is regulated by valves. The postcapillary sacks joined the lymph vessels proper the media of which was built up of a muscle cell sheath. In these artery-type lymph vessels proper the lumen was found to be varying in diameter; some vessels seemed to have practically no lumen while other segments were open. Under the electron microscope thin, actin-type and thick, myosin-type myofilaments were seen in the muscle cells of lymphatic post-capillaries and lymph vessels proper. The lymph is accumulated in the postcapillary sack-like sinuses and from there is transported towards the centre by the contractions of artery-type lymph vessels. Thus, these structures may function as "lymph hearts" in the oedematous skin.