The myocardial isoenzyme of creatine kinase (MB CK) has been demonstrated in the serum of seven of 16 patients presenting with acute subarachnoid haemorrhage. All patients with elevated MB CK levels exhibited at some stage either intracranial arterial spasm as demonstrated by cerebral angiography, or a focal reversible neurological deficit not due to direct pressure by either intracranial haematoma or aneurysm. The presence of MB CK isoenzymes in sera of patients with subarachnoid haemorrhage may be an index of cerebral ischaemia, and may thus be of value in determining optimal timing of surgery or, ultimately, the long-term prognosis. Changes in the sympathetic nervous system may be a common factor producing both MB isoenzyme release from the myocardium and intracranial arterial spasm.