Intraperitoneally (i.p.) administered iloprost produced a writhing response indicating nociception. This effect induced by 4 micrograms/kg iloprost was dose dependently protected by morphine with an ED50 (95% confidence limits) value of 0.039 (0.0018-0.067) mg/kg. On the other hand, indomethacin had no effect on iloprost-induced writhing. Thus, this effect of iloprost seems to be relatively more sensitive than other irritants-induced responses on determining the analgesic potency of narcotic drugs.