The concentration of medroxyprogesterone acetate (MPA) in plasma samples taken 24 h after intramuscular or oral administration of 100 mg daily doses of the drug to patients with endometrial adenocarcinoma was measured by radioimmunoassay. In general, a plasma level of about 4 ng/ml was found 24 h after the dose, independent of the route of drug administration. However, in three of the patients to whom intramuscular MPA was given, considerably higher values were found. A maximal plasma level was achieved three hours after ingestion of 100 mg MPA. This was followed by a rapid decline to 20-25% of the peak value after about 12 h. A rather small day-to-day intraindividual variation was found in daily blood samples taken just before administration of the next dose. However, considerable differences were found between individuals and it is concluded that this variation in plasma levels may be reflected in the clinical efficacy of the treatment. Thus further studies in which plasma values and clinical effectiveness are correlated seem to be indicated.