A pharmacokinetic analysis of propylthiouracil (PTU) plasma concentration versus time data for twelve euthyroid subjects was performed using the NONLIN computer program. The plasma concentration-time curves were best described by a one-compartment open model. PTU was administered to fasted subjects as a single 150-mg peroral dose. Venous blood samples were taken for a period of 8 hrs, and the concentration of unmetabolized PTU was determined by a new specific high-pressure liquid chromatographic procedure. Estimates of pharmacokinetic parameters generated in the study were compared with corresponding parameters reported in the literature.