1. Plasma concentrations of cerebral metabolic activating drugs (bifemelane, indeloxazine and propentofylline) were studied in 68 patients (male 25, female 43) with dementia or other organic brain diseases. 2. The variations in plasma concentrations of these drugs were much bigger than expected. Measurements of bifemelane level with time course also disclosed that the concentrations were relatively stable for several months, but they varied very much among patients. 3. These findings suggest that drug monitoring are important in terms of evaluation of drug efficacy and prevention of side effects.