Electrocardiograms (ECG) and laboratory data were studied in 130 neurosurgical patients before and after neurosurgery. The following results were obtained: 1. The postoperative ECG showed ST-T changes in 21 % of the cases with subarachnoid hemorrhage, 15 % with cerebral tumor, 37.5 % with supracellar tumor and 50 % with Parkinson syndrome. 2. In 50 % of subarachnoid hemorrhage and 47 % of cerebral tumor, postoperative ECG deteriorated within 1 week. After 2 weeks such deterioration was observed in only 37 % in the former cases and 43 % in the latter. 3. Postoperative ECGs were improved or unchanged in 83 % cases of cerebral bleeding, 75 % (within 1 week) and 83 % (after 2 weeks) of intracerebral bleeding, and in all of Parkinson syndrome. However, in the cases of supra-cellar tumor, postoperative ECGs deteriorated 100 % within 1 week and 75 % after 2 weeks. 4. Serum K(+) and Ca(++) were decreased significantly in the group with deteriorated postoperative ECGs. The R. B. C., Hb and Ht were decreased regardless of postoperative ECGs. 5. Postoperative ECGs deteriorated when the operation time lasted longer than 6 h or blood loss during the operation was over 1,000 ml.