Radioactive human serum albumin (RHSA) and urea were injected into the pleural cavity of patients with pleural effusions. The dynamics of the pleural liquid was investigated based on the volume of the pleural liquid and on the change in the concentrations of markers. RHSA concentration in the pleural liquid decreased linearly and slowly. Urea disappeared rapidly and exponentially. The analysis shows that water exchanges rapidly at the pleural capillary and that the volume of the pleural liquid is an affecting factor on the turnover of the pleural volume. The dynamics of malignant effusion was the same as that of tuberculous effusion.