Patient-controlled analgesia (PCA, intravenous self-application of narcotics) was studied during the early postoperative period. Subjects were 40 ASA I-III patients recovering from elective major and minor surgery (each 20 having undergone abdominal or orthopaedic operations). Morphine boluses each of 1.92 mg were available via a hand-button whenever the patients felt pain relief necessary, and delivered by a microprocessor-controlled injection pump (On-Demand Analgesia Computer, ODAC). Hourly maximum dose was set to 15 mg with a pump refractory time of 1 min between valid demands. A continuous low-dose morphine infusion (0.23 mg/h was additionally administered in order to prevent catheter obstruction. Duration of the PCA period was 20.3 +/- 4.3 h (mean, standard deviation). During this time, 18.3 +/- 10.6 demands per patient were recorded resulting in individual morphine consumption of 29.6 +/- 16.6 micrograms/kg/h. Self-administration was characterized by considerable intra- and inter-individual variability. Following abdominal surgery, significantly more morphine was needed compared with orthopaedic patients, although pain relief was found less in the former group. There were no statistically significant differences in drug requirements or pain scores between the sexes. With respect to different anaesthetic techniques, patients recovering from spinal anaesthesia needed lower morphine doses only during the first few (1-4) h in comparison with neuroleptanalgesia. Over-all efficacy and patient acceptance proved to be excellent. Effectiveness of PCA was judged superior by about 84% of patients when compared with previously experienced conventional postoperative analgesia.(ABSTRACT TRUNCATED AT 250 WORDS)