Of 66 patients undergoing hypotensive anaesthesia for oto-rhino-laryngological surgery, a sufficient low-pressure state was obtained in only 72% of patients when nitroglycerine was used. The cumulative nitroglycerine-consumption varied between 4.0 and 98.6 mg (0.05-1.698 mg/kg body weight). In 40 cases a normal or weak nitroglycerine-effect was accentuated by dihydralazine and the extend and time course of the interaction were investigated. The exceedingly marked additional effect of this substance indicates a mutual potentiation of both drugs. The maximum of the dihydralazine-action appeared after 25-35 minutes. In 16 further cases dihydralazine was given intravenously before the nitroglycerine infusion as a premedication, whereby always constant and satisfactory hypotension could be produced. A disadvantage of these combinations, occurring in several cases, was the decreased manoeuvrability of hypotension. The continuation of the long-term sodium nitroprusside-hypotension by using nitroglycerine was succesful. Even after nitroglycerine application longer than 3 hours (up to 6 hours) the acid-base-status remained normal.