1. Converting enzyme and neutral endopeptidase activities were both measured every 3 h by a fluorimetric method in plasma and cerebrospinal fluid of patients diagnosed as migraineurs with aura after lumbar puncture, which was performed 9 h after an acute oral dose of enalapril or placebo.
2. A reduced converting enzyme activity, as compared with placebo, was observed in patients who were given enalapril. On the other hand, neutral endopeptidase activity detected after enalapril did not differ from that measured after placebo.
3. The results seem to indicate that enalapril penetrates the blood-brain barrier in sufficient amounts to reduce converting enzyme activity. Moreover, neutral endopeptidase was not affected by enalapril. Therefore, those clinical effects of the drug which have been attributed to the involvement of central opioid mechanisms may depend on the inhibition of brain converting enzyme but not the inhibition of brain neutral endopeptidase.