Clinical observations have shown, that administration of Co-trimoxazole (Trimethoprim-Sulfamethoxazole) may result in an elevation of the serum-creatinine level. In 27 subjects the endogenous creatinine- and inulinclearance were measured before and during oral or parenteral application of Co-trimoxazole. The inulinclearance showed no significant differences (p greater than 0,05). But the endogenous creatinine clearance showed a reduction of 18% during the oral and of 13% during the parenteral treatment (p less than or equal to 0,05). These observations indicate a functional inhibition of the tubular creatinine secretion by Co-trimoxazole.