In this study, the effects of lead acetate on two types of pain (nociception and inflammation) induced by formalin and its interaction with the opioid system and morphine-induced analgesia were examined. Male albino mice weighing 22-27 g were used in the experiments. Morphine was administered subcutaneously 30 minutes before formalin injection. Lead acetate was intraperitoneally administered 90 minutes before any injection. Different doses of morphine induce antinociception in both phases of the formalin test. Lead acetate induced no does-dependent nociception in the early phase, but caused dose-dependent analgesia in the late phase. Pretreatment with lead acetate antagonized the effect of morphine in the early phase. On the other hand, the effect of lead acetate in the early phase was reduced by morphine and its effect was eliminated in the late phase. We conclude that lead acetate can modulate the pain response and interact with morphine-induced antinociception. Additional research is suggested to identify the mechanisms of these effects.