An on demand intramuscular analgesic system using the Cardiff Palliator was tested. Forty consenting patients were studied after cholecystectomy in a double blind trial using increments of buprenorphine (0.15 mg), meptazinol (50 mg), morphine (5 mg), and pethidine (50 mg). Most patients attained good levels of pain relief (mean analogue pain score 36.5), comparable to intravenous on demand analgesia. There were no technical complications. Significant differences were found between drugs in dizziness (pethidine produced the worst score) but not with other side effects. Buprenorphine produced longer lasting analgesia than meptazinol or pethidine and also gave the lowest pain scores. Patterns of analgesic consumption were the same as with intravenous on demand systems, but larger amounts of drug were generally used. Relative analgesic potencies derived from drug consumption rates were also consistent with those from intravenous on demand studies. An on demand intramuscular analgesic system offers a simple but effective means of relieving severe postoperative pain.