The effect of lithium on the antinociceptive action of morphine and two tricyclic antidepressants, imipramine and protriptyline was studied with hot plate and phenylquinone writhing tests in mice. alone had no significant antino- Lithium ciceptive effect but it increased morphine analgesia with both methods studied and decreased the antinociceptive effect of tricyclic antidepressants, especially that of protriptyÜne, in the hot plate method. This result supports the view that the mechanism of analgesic action of morphine may be different from that of tricyclic antidepressants.