Skin electrical resistance is determined by the degree of sweating of the skin which is, in turn, related to sympathetic nervous system activity in the area concerned. It is increased when the nerves supplying the area are damaged or blocked by local anaesthetic agents. We have assessed the temporal and spatial relationship between the onset of sympathetic and sensory loss in the hand following brachial plexus block in 44 patients. Skin electrical resistance, measured using a simple ohm meter, has been shown to increase within 2 min of brachial plexus blockade with 1% lignocaine and adrenaline 1:200,000. This increase is an early and reliable indicator of subsequent, and occasionally delayed, sensory loss.