74 spontaneous seizures in 26 patients with a clinical diagnosis of temporal lobe epilepsy (complex partial seizures) were recorded by simultaneous ambulatory cassette monitoring of the electrocardiogram and the electroencephalogram (EEG). In 24 patients (92%) seizures were associated with an increased heart rate. The maximum heart rates exceeded 120 beats/min in 67% of seizures, 140 beats/min in 30%, and 160 beats/min in 12%. The acceleration of heart rate was greater in patients under than in those over 25 years old (p less than 0.01) and in those not treated with than in those on anticonvulsant drugs (p less than 0.01). Ictal cardiac arrhythmias occurred in 42% of the patients and the commonest was an irregular series of abrupt rate changes which occurred towards the end of the EEG seizure discharge. Asymptomatic (clinically silent) arrhythmias occurred no more frequently in the patients than in age and sex matched healthy subjects. These secondary autonomic effects of epilepsy may lead to diagnostic errors if their cerebral origins are not suspected. They seem to be reduced in severity by anticonvulsant drugs (ACD) and they may account for sudden unexplained deaths in epileptics.