The neuromuscular block induced by AH 8165 D, a new non-depolarizing muscle relaxant, has been studied in two series of anesthetised patients. All the anesthetics were induced by Althesin, and intubations performed under local anesthesia. The first series of 15 patients received only 75% nitrous oxide and 25% oxygen for the first hour of anesthesia. The second series received in addition 1 to 1.5% of enflurane. After an hour all the patients received a single dose of AH 8165 D. Dose-response curves were established in both series and it was found that without enflurane the ED50 of AH 8165 D was 8.29 mg/m2 and with enflurane 4.07 mg/m2. Therefore enflurane potentiates the action of AH 8165 D by 2.03 times, although it is impossible from this study to indicate precisely how and where enflurane can affect neuromuscular transmission.