The present study shows that Ukrain, administered i.p. in doses of 9.5 and 19 mg/kg once daily for 10 days in mice, produced evident antinociceptive action in the writhing syndrome and hotplate tests. Moreover, we found that as a result of 10-day treatment with Ukrain the antinociceptive action of morphine (0.1 mg/kg s.c. administered after the last dose of Ukrain) was significantly decreased. These results and the observed interaction between Ukrain and morphine seem to be contraindications to their combined usage.