The effect of oral propranolol on the abnormal ECGs of six cases of subarachnoid haemorrhage (SAH) was investigated. The P wave peaking seen in three of the six cases was abolished. The short P-R interval observed in three of the six cases was lengthened, one markedly so, and seemed independent of changes in heart rate. T wave flattening seen in one case was abolished, while T wave inversion seen in one other was unaffected. T wave peaking seen in one case was increased with propranolol, but abolished by atropine. Tall U waves seen in two cases were diminished in amplitude and the Q-Tc (which was abnormally long in four) was shortened in all cases. Propranolol had no effect on the pathological Q waves which appeared in one case. It was concluded that these findings support the suggestion that P wave peaking, a short P-R interval, tall U waves and a long Q-Tc, often seen in ECGs of cases with SAH, are catecholamine-induced. Interplay between sympathetic and parasympathetic activity occurs in SAH with T wave peaking resulting from increased vagal tone. Catecholamine-induced cardiotoxic effects can cause ECG patterns mimicking myocardial infarction and may, in some cases, contribute to increased morbidity and mortality. There is thus a need for a controlled blind trial of prophylactic oral propranolol in SAH.