The postoperative analgesia afforded after colonic surgery by IV opioid, clonidine and lignocaine given intra- and postoperatively was evaluated. In a double-blind randomised trial, 80 male patients scheduled for colonic resection under general anaesthesia received fentanyl 5 micrograms.kg-1 at induction and another 4 micrograms.kg-1 before skin incision (group A) or fentanyl (same dose) plus clonidine 4 micrograms.kg-1 in 20 min + 2 micrograms.kg-1.h-1 (group B, C) or fentanyl plus clonidine (same dosage) plus lignocaine 2 mg.kg-1 before skin incision, repeated before peritoneal incision and retractor placement (group D). In the four groups, intraoperative boluses of fentanyl 2 micrograms.kg-1 were given in response to the painful stimulation of the procedure. Postoperative pain was managed with PCA delivering 2 mg morphine per request in group A, 1.5 mg morphine in group B, 1.5 mg morphine + 15 micrograms clonidine in group C and 1.2 mg morphine + 15 micrograms clonidine + 23 mg lignocaine in group D. Postoperative analgesia was assessed by recording the analgesic demands (met and unmet) and the dose of morphine delivered at 6, 12, 18, 24, 36 hours. Side-effects, pain and sedation analogue scores were also recorded. Analgesic demands and delivered morphine dose were reduced, at any time interval considered, in groups B, C, D, compared with A (P < 0.001). No differences were noted between the group B, C, D. Pain analogue scores were better in groups B, C, D compared with group A (P < 0.001). Sedation and side-effects were not increased in groups B, C, D.(ABSTRACT TRUNCATED AT 250 WORDS)