The haemodynamic effects of intravenous mexiletine have been studied in 16 patients with valvular heart disease without clinical evidence of heart failure. A bolus injection of 150 mg administered to 6 of the 16 patients resulted in a mean plasma concentration above the therapeutic range for at least 5 minutes after the drug was given. A small but significant rise in the mean pulmonary artery pressure occurred. In 10 patients, the effects of intravenous mexiletine were compared with those of intravenous saline in a double blind trial. No significant difference was found in the haemodynamic effects, though both saline and mexiletine produced a small rise in the mean pulmonary artery pressure. Mexiletine when administered to patients without heart failure in doses known to be clinically effective did not have important adverse haemodynamic effects.