Quantitative assessment of extradural bupivacaine analgesia. Bupivacaine (0.5%) 20 ml was administered extradurally to six healthy volunteers. It was found that simultaneous application of 10 needles to the skin could evoke pain when analgesia was obtained to one needle stimulation. In addition, a laser beam was used as a quantitative technique to activate simultaneously many cutaneous nociceptors. For 7 h, thresholds (sensory and pain) and pain-evoked brain potentials (amplitude and latency) to laser stimulation were monitored and used for quantitative assessment of onset, efficacy and duration of analgesia at various dermatomes (C7, T8, T10, T12, L1, L3, S1). The onset time of analgesia was shortest and conduction delay longest at the dermatome related to the site of injection (L3). Full analgesia was obtained at L1, L3 and S1, although the peak efficacy at S1 was delayed for 120-180 min after injection. A minor effect was found at dermatome C7 approximately 60 min after injection. 