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It has been shown that an active metabolite- morphine-6-glucuronide-is formed in man after administration of morphine. In spite of the chemical variation, the glucuronide still possesses high affinity to opioid receptors and the polarity of the glucuronide does obviously not exclude penetration of the blood-brain barrier. After peroral administration of morphine the concentration of the metabolite rises and exceeds within less than 30 minutes the concentration of the parent compound. The affinity profile of morphine-6-glucuronide as measured by binding experiments in membrane homogenates deviates from that of the parent compound. It can not be excluded at the moment that morphine-6-glucuronide will prove superior to morphine for long term treatment, since the kinetic behaviour excludes sharp undulations of the plasma concentration and probably of the concentration at receptor sites in the brain. It is speculated that this type of pharmacokinetic behaviour will reduce tolerance phenomena and abuse liability.