Flecainide acetate has been shown to be a potent antiarrhythmic agent which is active for more than 8 h, whether given intravenously or orally. However, the negative inotropic effect demonstrated in animal studies could hamper the potential clinical utility of the drug. Ten patients with coronary artery disease but without cardiac failure were given intravenous flecainide (2 mg/kg). Stroke index (SI), left ventricular systolic pressure (LVP), end diastolic pressure (EDP) and LV contractility indices (max dP/dt, VCE 40 mm Hg, peak VCE, Vmax from total pressure (TP] were measured immediately before and 10 min after flecainide, under resting conditions and during atrial pacing with heart rates up to 133 +/- 4.2 beats/min (mean +/- s.e. mean). It is demonstrated that flecainide has a negative inotropic effect, not only under resting conditions, but also less apparently during pacing-induced tachycardia. The effect appears to be dose-related and may result in a reduction of cardiac performance.