One investigator at each of 4 institutions followed the requirements of a core protocol designed to evaluate propoxyphene napsylate (50, 100 and 150 mg), fenoprofen calcium (200, 400 and 600 mg) and their combination (50/200, 100/400 and 150/600 mg) in patients reporting postpartum pain. Placebo and aspirin (650 mg) were included as control medications. Analyses were based on subgrouping which isolated individually the interactions between medications and the following factors: dose-observation criteria, the investigator-observer-institution differences, the intensity of pain at the time the medication was given, and postepisiotomy-wound compared to uterine-cramp pain. Each of these factors influenced the absolute scores for analgesia significantly, but had no significant influence on the relative rankings of the medications. The analgesia scores for individual patients in each subgroup were transformed to ridits and then pooled. A linear increase in effectiveness occurred in response to increasing doses of propoxyphene, fenoprofen, and their combination in this dose range. The dose responses were essentially parallel. The combination was more effective than either drug alone.