The effect of lidoflazine administration (120 mg t.i.d. for 9 weeks) on work tolerance (bicycle ergometer), frequency of anginal attacks, and nitroglycerin consumption was investigated in 28 male patients with stable angina pectoris in a combined single-blind/double-bline study. Lidoflazine increased work tolerance and reduced the frequency of anginal attacks and nitroglycerin consumption. The higher work tolerance level was reached at maximal heart rate and heart rate--systolic blood pressure product values similar to those before treatment. The values of these variables after 3 min of exercise at 60 W, however, were significantly lower after treatment with lidoflazine. These findings indicate that the heart is performing more economically during lidoflazine treatment. The improved work tolerance can probably be ascribed to lidoflazine and not to a training effect because of the significant reduction of this variable in the patients allocated to placebo as compared to those remaining on lidoflazine treatment. The side effects were generally slight. In one patient the prolongation of the QT interval due to lidoflazine resulted in rhythm disturbances.