The intracerebrally administered dynorphin-B produced not only an antinociceptive response but a motor dysfunction such as "barrel-rolling", circling, jumping or ataxia in mice. These two effects were observed in a same dose range. Both responses were also produced by a nonopioid fragment, des-Tyr-dynorphin-B. Bestatin, an aminopeptidase inhibitor, markedly potentiated the antinociceptive response induced by dynorphin-B but not the motor dysfunction. Bestatin did not affect the responses produced by des-Tyr-dynorphin-B. In the presence of bestatin, low doses of dynorphin-B produced an antinociceptive response without the motor dysfunction. Naloxone antagonized the potentiated antinocicpetion but had no effect on the motor dysfunction. These results suggest that dynorphin-B produced an analgesia through opioid receptors and that this peptide also induced a motor dysfunction through a nonopioid receptor.