We have compared 0.5% bupivacaine 75 mg (group A; n = 15) with three 0.5% bupivacaine 75 mg-ketamine mixtures for extradural block in 59 ASA I-III patients undergoing total knee replacement in a randomized, double-blind study. The following doses of preservative-free 1% ketamine were used: 0.3 mg kg-1 (group B: n = 14); 0.5 mg kg-1 (group C: n = 5); and 0.67 mg kg-1 (group D: n = 15). Level of sensory block, degree of motor weakness and sedation scores were recorded before and after operation. Duration of postoperative analgesia was also noted. There was no difference between groups in median maximum level of sensory block (group A: T4 (range T10-T2); group B: T4 (T10-T2); group C: T4 (T8-T2); and group D: T3 (T8-C3)) or in the degree of motor block. Thirty-three of the 44 patients who received ketamine showed signs of systemic absorption (blurred vision, sedation) within 10 min of injection. There was no significant difference between groups in median duration of analgesia (group A: 240 (range 115-340) min; group B: 198 (97-460) min; group C: 150 (122-448) min; and group D: 210 (130-390) min). No patient suffered any adverse psychomimetic effects. We conclude that at the doses used, addition of ketamine to extradural bupivacaine did not improve extradural block in adult patients undergoing total knee replacement.