After oral administration of 100 mg Prednimustine to patients with metastatic breast carcinoma, prednisolone levels and the area under the plasma concentration-time curve between 2 and 24 h are approximately half of those obtained after the administration of an equivalent dose of prednisolone. As previous studies have reported up to 55% of unchanged Prednimustine in feces. It is suggested that prednisolone plasma levels are as good a parameter of Prednimustine absorption and bioavailability as chlorambucil levels, Prednimustine itself having never been detected in plasma. Whether Prednimustine is hydrolyzed in the intestine before absorption or rapidly taken up by the tissues and hydrolyzed in situ after absorption is unclear at the moment. Myelosuppressive effects of Prednimustine correlated very well with maximal prednisolone levels, emphasizing the value of prednisolone level measurements.