Prednisone and prednisolone extraction was determined during seven sessions of plasma exchange in four patients. Each patient received 0.5 mg/kg of prednisone administered orally 2 hours before plasma exchange. Concentrations of prednisone and prednisolone were determined in blood before, every half hour during and at the end of plasma exchange. Both drugs were also determined in removed plasma, mean volume of removed plasma being 0.96 plasmatic mass. The amount of combined prednisone and prednisolone removed by plasma exchange was 1.70% of the administered prednisone dose. This quantity appears minimal and supplemental dosing of prednisone or modification of administration time would seem to be unnecessary.