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Ten thyrotoxic individuals, who otherwise had no evidence of cardiovascular disease, underwent continuous ambulatory 24-hour ECG monitoring, before and after antithyroid treatment. The mean age of the subjects was 41 ± 14.4 years (mean ± SD) with a range of 22 to 66 years. When subjects were thyrotoxic, the mean heart rate for the group was 104 ± 10.8 bpm. This fell to 82 ± 6.8 bpm when the subjects were rendered euthyroid (p < 0.001). Circadian rhythm of heart rate response was maintained in the thyrotoxic state, although heart rate variability was significantly increased (p < 0.001). The prevalence of ventricular premature contractions was not significantly different before and after treatment, although premature atrial contractions were more prevalent during the middle third of the day (p < 0.01) when subjects were euthyroid. These findings support the view that normal adrenergic responsiveness persists in hyperthyroidism, and for most individuals treatment does not significantly alter the prevalence of ectopic activity.