The analgesic, anti-inflammatory and gastrointestinal effects of rimazolium, morphine and prostaglandin synthesis inhibitors were compared in a battery of tests. Rimazolium, morphine and indomethacin all inhibited carrageenin-induced inflammation; however, the onset of action was different. The first (histamine-serotonin) phase was inhibited by rimazolium, the second (kinin) phase by morphine and the third (prostaglandin) by indomethacin. The chemoluminescence of leucocytes was inhibited by morphine and indomethacin but was unaffected by rimazolium. Prostaglandin-mediated pain (ACh, ATP, acetic acid writhing) was inhibited by all three types of compound; however, pain reaction where prostaglandins (PGs) are not involved (MgSO4 writhing) was inhibited by rimazolium and morphine, but not (or only slightly) by PG synthesis inhibitors. Gastric lesions produced by indomethacin were depressed by rimazolium and aggravated by morphine. These results suggest different mechanisms of anti-inflammatory and analgesic action of rimazolium, morphine and PG synthesis inhibitors.