Onset phase of spinal bupivacaine analgesia assessed quantitatively by laser stimulation. Analgesia was assessed quantitatively at various dermatomes (C7, T8, T10, T12, L1, L3, S1) for the first 30 min after subarachnoid administration of 0.5% bupivacaine 3.5 ml. Stimulation with 10 needles and laser stimulation could evoke pain in dermatomes with adequate analgesia to single needle stimulation. Analgesia was assessed by thresholds (sensory and pain) and by pain-related brain potentials (amplitude and latency) to laser stimulation. Little analgesia was found at T10, but it increased gradually towards caudal segments. The dermatome related to the site of the injection (L3) was not blocked to a greater extent than the surrounding dermatomes. Conduction time (the latency of the evoked brain potential) was increased relatively more from the S1 dermatome compared with L1. 