Thirteen patients, who underwent surgery for myocardial revascularization, were given neostigmine, 0.25-1 mg i.v., in order to control intraoperative sinus tachycardia. Three minutes after the administration, the drug was effective in all patients, by decreasing the heart rate and improving the indices of the risk of myocardial ischemia. Seven minutes later the heart rate decreased further. In 5 patients out of 13, short periods of bradycardia were observed during surgical compression or dislocation of the heart. As cardiac rate decreased, P and T waves and ST, QT, and QTc intervals lengthened without reaching pathological levels. Three minutes after neostigmine, the stroke volume was unchanged in spite of the decrease of the heart rate; consequently cardiac index decreased. Seven minutes later a slight increase of the stroke volume balanced the further decrease of the heart rate; so cardiac index did not change any more. Arterial pressure decreased slightly after neostigmine whereas systemic vascular resistance was unaffected. Also central venous pressure did not change while pulmonary capillary wedge pressure showed a small increase 10 minutes after neostigmine. Finally airway resistance did not change significantly.