In 32 patients undergoing spinal anaesthesia with lidocaine or lidocaine plus adrenaline, the concentration of lidocaine base was measured in repeated venous blood samples. Half of the patients were given 100 mg lidocaine alone and the other half 100 mg lidocaine with the addition of 0.2 mg adrenaline. The blood concentration of lidocaine was significantly higher in the lidocaine group than in the group that received lidocaine with adrenaline. The mean maximal concentrations in these two groups lay between 0.2 and 0.3 and between 0.1 and 0.2 micrograms/ml, respectively (P less than 0.01). The time required for the individual maximal concentration to be reached was independent of whether the anaesthetic contained adrenaline or not. The blood concentration showed no correlation to body weight, height, or body surface area. In the first 60 min after the spinal injection the extent of the sensory block was the same in the two groups, but at 120 min this extent was significantly greater in the lidocaine-adrenaline group (P less than 0.01). The results show that the blood concentration of lidocaine is low in spinal anaesthesia compared with other types of regional anaesthesia.