The thoracic duct and right lymphatic duct of the dogs in the experimental group were ligated at the neck. Saline labelled with Evans blue was injected (10 ml.kg-1) into the pleural cavity. The colloid osmotic pressure of the saline was adjusted to be exactly equal to that of normal pleural liquid. The change in the volume of the liquid and the decrease of the marker were studied for 3 h. The dynamics of the pleural liquid were investigated. In the control group, the volume of the pleural liquid decreased at the rate of 1.31 ml.h-1.kg-1 body weight and 87% of it left via the lymphatics. With the ligation of the bilateral lymphatic ducts, the lymphatic removal rate decreased to about one tenth. This result shows that the hydrothorax leaves the pleural cavity mainly via the lymphatics.