Postoperative pain relief is only apparently an easy task. A brief survey of literature investigating the discomfort experienced in the postoperative phase is all but encouraging. One can identify four basic problems in obtaining adequate results by delivery of analgesic drugs: a) the biological variability among individual patients, b) the unpredictable uptake of the drug administered intramuscularly, c) the time lag involved between request by the patient and the subsequent administration of a single dose and d) the lack of knowledge about the nature of the discomfort and its remedies. An alternative strategy introducing "On-Demand" analgesia administering prescribed doses at the right moment is presented and analysed a) as an operant conditioning process implementing a particular reinforcement schedule (behavioural sciences), as well as b) a negative feedback control loop that entrust the central judgement to the patient (system theory). Both approaches give insight into the results: the technique copes with biological variability; anticipating pain induced by fear disappears; the feedback strategy works well and patients adapt to a wide range in prescriptions; intermittent administration makes more efficient use of the analgesic; an optimal result is demonstrated in studies comparing on-demand analgesia with the normal IM-regime and epidural analgesia; continuity in pain relief can be obtained in routine clinical practice.