The efficacy of an extradural infusion of 0.0625% or 0.125% bupivacaine was studied in 98 primigravid mothers in active labour. No special measures were taken to posture the mother (except to avoid aorto-caval compression). The study regimen included a control group (no infusion) receiving intermittent top-ups (0.25%. bupivacaine 8-10 ml), two groups receiving bupivacaine 6.25 mg h-1 infusion in different concentrations (0.0625% and 0.125%), a fourth group receiving 0.125% bupivacaine 12.5 mg h-1 infusion, and a fifth group receiving 0.125% bupivacaine 18.75 mg h-1 infusion. The optimum infusion rate was 0.125% bupivacaine 10 ml h-1, at which 69% of primigravid mothers required none or only one "top-up" of 0.25% bupivacaine 8-10 ml during a mean duration of 7.1 h labour. In the group who had no extradural infusion, only 32% of mothers managed with one or no top-up. The median interval between top-ups was increased from 145 min in the no infusion group to 245 min in those mothers receiving 0.125% bupivacaine 10 ml h-1 by infusion. Increasing the rate of infusion to 15 ml h-1 did not improve the results. Spread of local anaesthetic to higher levels was limited (less than T5) so that testing sensory loss at the T5-6 level at 2-hourly intervals should detect accidental spinal blockade resulting from inadvertent intrathecal infusion.