The uptake of 14C-labeled aminoglutethimide into the brain was investigated in the rat after intracarotid injection according to the method of Oldendorf, as well as in cisternal cerebrospinal fluid obtained by suboccipital puncture after i.v. injection of the drug. The brain uptake index of aminoglutethimide after intracarotid injection was 30.7 + 2.4%. Cerebrospinal fluid/blood quotients after i.v. drug injection were 0.53 at 10 min and 0.26 at 60 min. The results of both methods clearly show that aminoglutethimide easily penetrates the blood brain barrier.