The elimination of para-aminosalicylic acid (PAS) after the intravenous injection of 20 mg PAS sodium/kg was estimated in patients with liver disease, in uremic patients and in volunteers without damage of the liver or kidneys. The drug was estimated with a colorimetric and fluorometric method. In the volunteers, the half-lives obtained with the fluorometric method were significantly longer than those estimated with the colorimetric method. This is caused by the estimation of more PAS metabolites by the used fluorometric method. In the patients with renal insufficiency (dialysis patients) the elimination rate of unchanged PAS – estimated with the colorimetric method – was not altered, whereas the elimination of PAS and its metabolites extractable by ethyl acetate was markedly slowed in comparison with the results obtained with the volunteers. The clearance of the unchanged PAS was even increased in the uremic patients. The serum protein binding of PAS was lowered significantly in the serum of uremic patients. In patients with liver cirrhosis, acute virus hepatitis and cholangitis the elimination rate of the drug was not altered in comparison with the volunteers. The results show that the dose of PAS in patients with renal insufficiency may not be reduced. The therapeutic level of the drug cannot otherwise be reached in these patie