We have studied the effect of extradural compliance and extradural resistance on the spread of extradural analgesia. In 111 patients aged 21-75 yr, compliance and resistance of the extradural space were calculated by a mathematical analysis (using the Windkessel theory) of the extradural pressure-response curve to injection of a given volume of local anaesthetic. The calculated mean extradural compliance was 0.39 (SD 0.13) ml mm Hg-1 and this increased with advancing age (P less than 0.01). The total number of analgesic segments blocked was related to extradural compliance (P less than 0.01). Segmental dose requirement was related inversely to extradural compliance (P less than 0.01). Calculated extradural resistance was 26.8 (14.5) mm Hg s ml-1 and this decreased with advancing age (P less than 0.05). The total number of analgesic segments blocked was related inversely to extradural resistance (P less than 0.05). Segmental dose requirement was related to extradural resistance (P less than 0.05).