Abstract: K₃ was shown to be active in enhancing the ability of rats, mice and rabbits, to resist the noxious thermal stimulation and exhibited a dose-related analgesic effect. The potency of the analgesic effect of K₃ ip was equivalent to that of morphine HCl 4 mg/kg and 1 mg/kg sc as measured in mice tail flick and hot plate tests respectively.K₃ during 50 μg and 100 μg injected into the lateral ventricles elevated the pain threshold of rabbits similar to intravenous administration.The analgesic effects of K₃ given systemically to mice or injected into the lateral ventricles of rabbits were reversed by naloxone.K₃ potentiated the analgesic effect of morphine; erosstolerance was found between the analgesic effect of the two agents.In addition, K₃ was shown to inhibit the contraction of GPI by field stimulation, which was reversed partially by naloxone. It appeared that all of these effects of K₃ may be mediated through endogenous opiate like substances or opiate receptors. K₃ 32 mg/kg ip twice a day for 5d in mice induced tolerance to the drug. K₃ 100 mg/kg ip for 14 times within 7d in mice and then challenge with ip naloxone 6 mg/kg, no jumping response was observed as seen with morphine.