Aspirin and codeine, standard reference analgesics, are frequently used as positive controls in clinical trials of new oral analgesics. In randomized parallel double-blind studies, single doses of aspirin and codeine were compared with placebo in episiotomy pain (99 patients) and in postpartum uterine pain (130 patients), common models in analgesic trials. With aspirin, 600 and 1,200 mg, in episiotomy pain, analgesia as measured by pain intensity difference (PID) scores began within 1 hr, peaked at the second hour (p less than 0.01), and continued to the fifth hour (p less than 0.01). In uterine pain, responses with aspirin, 650 mg, were observed to be equally good. With codeine, 60 mg, in episiotomy pain measurable analgesia was present by the second hour and was significant at the fourth hour (p less than 0.05); in uterine pain, responses were indistinguishable from placebo throughout an 8-hr time-course. Codeine seemed ineffective and therefore umacceptable as a positive control in uterine pain. These data imply that the two postpartum pain models are qualitatively different: episiotomy pain seems sensitive to both aspirin and codeine, while uterine pain appears sensitive to aspirin but not to codeine.