Fentanyl 50 micrograms, bupivacaine 50 mg and two mixtures containing fentanyl 50 micrograms and bupivacaine 25 mg or 12.5 mg (0.25 and 0.125%), respectively, in a volume of 10 ml were administered via thoracic extradural catheters to 24 patients after major abdominal surgery. All patients received all four treatments, in a randomized order, so that each patient received one of the 24 possible combinations of the four treatments. Pain relief was assessed by a linear analogue pain scale and the Prince Henry Hospital pain score. The duration of pain relief, effects on ventilatory frequency, heart rate, arterial pressure and central venous pressure were also recorded. Mean reductions in the analogue pain scale for fentanyl, bupivacaine, and fentanyl in 0.25% and 0.125% bupivacaine were 80 (SEM 5) %, 87 (4) %, 86 (5) % and 77 (5) %, respectively (ns). Pain scores decreased by 62 (6) %, 83 (5) %, 77 (6) % and 72 (6) %, respectively (ns). Mean arterial pressure decreased to 90 (2) %, 70 (2) %, 81 (2) % and 82 (3) %, respectively, of pretreatment values. In this respect, bupivacaine alone was significantly different from the three other treatments (P < 0.001). Hypotension (reduction in arterial pressure greater than 25% of pretreatment mean arterial pressure) was also more frequent after bupivacaine alone (P < 0.01). Effects on ventilation, heart rate and central venous pressure did not differ between the four treatments.(ABSTRACT TRUNCATED AT 250 WORDS)