Lidocaine was infused at a fixed zero-order rate to eight healthy mongrel dogs until tonic extension (n = 5) and cortical seizures (n = 7) were produced. Lidocaine concentrations determined at 5-min intervals were used to calculate concentrations at which these effects occurred. The tonic extension phase occurred at a mean lidocaine concentration of 8.21 +/- 1.69 micrograms/ml. After a 1-month rest period, the same dogs were anaesthetized and ventricular tachycardia was produced by administering ouabain. Lidocaine was again infused at a fixed zero-order rate until all beats of ventricular origin were abolished. This occurred at a mean lidocaine concentration of 6.25 +/- 1.49 micrograms/ml.