After an oral single dose of 400 mg pipemidic acid administered to 7 patients with renal insufficiency of varying degrees of severity, serum concentrations were determined by microbiological and spectrofluorimetric methods. The results obtained revealed maximum values ranging between 1.8 and 7.2 micrograms/ml with a biological half-life of 5.7--16 h. Of some therapeutic significance was an increase in serum concentrations amounting to 2.6--12.8 micrograms/ml when pipemidic acid was administered at normal doses (2 X 400 mg/d) for 3 days. Under maintenance therapy maximum concentrations of microbiologically active substance in urine were found to range between 177 and 580 micrograms/ml. Even in severely impaired kidney function urine concentrations averaged 230 micrograms/ml, which indicates that therapeutically sufficient concentrations can be maintained by regular administration of normal doses at unaltered dosage intervals.