Total and unbound 2-(chlorophenoxy)-2-methyl propionic acid (CPIB) concentrations were measured in serum samples taken from patients on continuous clofibrate medication, over a 5- to 6-hour period during 1 day. All patients were on commonly accepted dosage regimens, and achieved effective levels of CPIB. Patients taking 1 g clofibrate twice daily showed a greater diurnal fluctuation of CPIB levels than did patients taking 0.5 g clofibrate three or four times daily.