In order to assess the effects of different doses of lignocaine on performance, nine healthy volunteers aged 21-34 yr received i.v. infusions of saline, low and high dose lignocaine (mean plasma concentrations 0.92 and 1.78 micrograms ml-1, respectively) in a double-blind randomized order. The Digit-Symbol Substitution Test (DSST) and Visual Analogue Scales (VAS) were performed repeatedly and a battery of performance tests once. The median (lower, upper quartile) number of correct responses for the DSST during the infusion period was: placebo 69 (67, 77); low 74 (71, 80); high 66 (61, 75) (P less than 0.001, General Linear Models; all pairwise comparisons P less than 0.05). None of the measures in the full battery showed any significant changes. VAS showed that subjects felt more interested (P less than 0.05), drowsy (P less than 0.01), dizzy, tense, abnormal, drunk and muzzy (P less than 0.001) with lignocaine than with placebo. These results confirm that lignocaine can produce acute performance effects (both improvements and impairments). Subjects were clearly aware of the presence of lignocaine, suggesting that subjective reports may be a useful indicator of its CNS effects.