During a 24-month period, 37 patients with fascicular blocks (RBB-LAH, alternating bundle-branch block, bundle-branch block with PR prolongation, RBBB-LAH with PR prolongation) were sheduled for operation. The study was undertaken to determine what effect the stress of anesthesia and operation might have on these patients, and to attempt to answer the question whether all patients with fascicular block should have demand pacemakers implanted. All these patients were in stable condition and had no history of transient complete heart block. Patients were monitored continuously intraoperatively and were followed by serial electrocardiograms to the third postoperative day. No instance of complete heart block was found. No significant problem arose both intra- and postoperatively. According to these findings, routine, prophylactic, temporary cardiac pacing in patients with fascicular blocks, before anesthesia and during operation, seems to be not necessary in the absence of cardiac symptoms.