This study was performed to investigate the effects of glycine on the development of tolerance to and physical dependence on morphine. Repeated administration of morphine (10 mg/kg) developed tolerance and physical dependence. Glycine (100, 200 and 400 mg/kg) was administered intraperitoneally (i.p.) to mice for 7 days prior to the morphine injection. Analgesic effects were estimated by the tail flick methods. The inhibitory degree of the development of morphine-induced analgsic tolerance by i.p. administration of glycine was evidenced by the increase in analgesic response to morphine. Glycine inhibited the development of tolerance to morphine. In addition, we separately measured jumping response as the naloxone-precipitated withdrawal sign in mice that had received the same morphine. Glycine reduced the number of jumping behaviors in morphine dependent mice. These results suggest that glycine might be useful the prevention or treatment of morphine tolerance and physical dependence.