1. The influence of long-acting propranolol and of atenolol on the circadian rhythm of heart rate was assessed using 24 h electrocardiographic recordings in elderly patients with essential hypertension. Eighteen elderly patients were investigated before treatment and after 12 weeks on 60 mg of long-acting propranolol once a day; 21 elderly patients were studied before treatment and after 12 weeks on 50 mg of atenolol once a day. The mean hourly heart rate in 24 h electrocardiographic recordings was used to fit cosine curves by the statistical technique of least squares, and the following parameters were estimated: mesor (rhythm-adjusted mean of the heart rate), amplitude (one-half of the total diurnal variation of the heart rate) and acrophase (the time when the heart rate was at its peak above the mean).
2. The cosine curves were fitted with a P value of 0.01 or less before and after treatment in all patients.
3. In the long-acting propranolol group, the mesor and amplitude were reduced significantly after treatment. The acrophase appeared significantly earlier after treatment than before treatment.
4. In the atenolol group, the mesor and amplitude decreased significantly after treatment. However, the acrophase did not change with treatment.
5. We conclude that both propranolol and atenolol decrease the mesor and amplitude through blocking cardiac β1-receptors, and that only propranolol may have some effects on the central nervous system, resulting in the circadian phase shift.