1. Structure-activity relationship studies with new semi-synthetic isomorphine derivatives revealed that substitution of an azido group in position 6 (azidomorphines) greatly increases the analgesic potency whereas tolerance and dependence liability tend to decrease. 2. Azidomorphine (6-deoxy-6-azidodihydroisomorphine) and 14-hydroxyazidomorphine (6-deoxy-6-azidodihydro-14-hydroxyisomorphine) being in animal tests 40-300 times more potent than morphine, are the most effective analgesics among the semi-synthetic morphine alkaloids. 3. As demonstrated on mice, rats and rhesus monkeys, a remarkable dissociation between the analgesic potency and physical dependence capacity was the result of the introduction of the 6-azido group into dihydroisomorphine. 4. A dichotomy between analgesic effect and tolerance and addiction liability was demonstrated with azidomorphine also in man and the new substance proved to exert significantly less untoward effects than either morphine or pentazocine. 5. Rymazolium (Probon) a new non-narcotic analgesic which strongly potentiates the analgesic and antagonizes the respiratory depressant effect of morphine alkaloids in animals proved to hinder the development of tolerance to morphine in animals and man. 6. The azidomorphine-rymazolium association was found to be less respiratory depressant than azidomorphine administered alone. In patients with chronic intractable pain, an association of azidomorphine (0.5 mg) and rymazolium (150 mg) achieved total pain relief without noticeable euphoria and none of the twelve patients showed, according to the Himmelsbach scoring system, acute abstinence syndromes after nalorphine administration.