Abstract. Twenty hyperthyroid patients were randomly assigned in a double-blind fashion to receive either nadolol 80 mg/day or placebo for 2 weeks; all patients then took carbimazole as well from 2–6 weeks. Twenty-four hour Holter ECG recordings at 0, 2 and 6 weeks showed that nadolol reduced the mean maximum heart rate by 19.9% (P < 0.0005) at 2 weeks and by 30.3% (P < 0.0005) at 6 weeks compared to 5.2% (ns) and 18.3% (P < 0.0005) in patients taking placebo. There was no alteration of the normal circadian variation of heart rate by nadolol. The minimum heart rate before therapy was significantly correlated with FT4 (r = 0.52) and with FT3 (r = 0.44). The percentage of time per hour during which the heart rate was greater than 100 was reduced by 79% at week 2 by nadolol compared to 22% in the placebo group. At the 6 week point the placebo group still had a tachycardia (mean maximum heart rate 101.6 beats/min ± 15.2 sd) compared to the nadolol group (80.4 ± 7.7). Nadolol did not cause excessive bradycardia. It is effective in the early management of hyperthyroidism and should be given for at least the first 4–6 weeks.