1. In a previous study in rats, an increased PAH clearance was found following chronic phenobarbitone administration. These results formed the basis for the present study in which fifteen healthy male volunteers were investigated and the parameters of liver microsomal enzyme activity and renal function were measured. 2. As parameters of liver microsomal enzyme activity, the antipyrine elimination in the plasma, the gamma-glutamyl-transpeptidase and the D-glucaric excretion in the urine were measured. Endogenous creatinine clearance, 51Cr-EDTA and 125I-Hippuran clearance were determined as measurements of renal function. 3. No correlation was found between any of the parameters of microsomal enzyme activity and renal function. 4. Of the fifteen volunteers, seven having a mean antipyrine half-life of 13.3 h were given antipyrine (500 mg) daily for 3 weeks. Afterwards all measurements of liver microsomal enzyme activity and renal function were repeated. The antipyrine half-life decreased to 8.5 h, while the antipyrine clearance was increased by about 56%. gamma-glutamyl-transpeptidase and D-glucaric acid were also significantly increased, while renal function remained unchanged. 5. Therefore, an increased PAH-clearance, as found in the rat, is not obtained in man following induction of liver microsomal enzyme activity.