A demand analgesia apparatus is described. It communicates with the patient by means of spoken messages in any desired language, so that preliminary instruction is unnecessary. Dosage is limited by a reduction in respiratory rate, and by a series of electronic fail-safe circuits. Operation of the system is illustrated in a series of 10 patients given fentanyl “on demand” for the first 12 h after operation. The mean dose rate 0.73±0.49 μg min−1. There was no evidence of cumulation or tolerance during the period of study.