2 Supine blood pressure was reduced in group A (11 patients) from 169.4 ± 5.06/111.2 ± 2.63 mmHg to 136.9 ± 2.55/90.9 ± 1.19 mmHg (P < 0.001) during the administration of atenolol alone. Concomitantly supine heart rate was decreased from 83.9 ± 4.10 beats/min to 59.7 ± 1.67 beats/min (P < 0.01) — 4th week. After the administration of atenolol over 8 weeks, supine blood pressure was 138.6 ± 1.21/94.4 ± 2.12 mmHg and supine heart rate was 59.5 ± 2.05 beats/min.

3 Supine blood pressure was reduced in group B (27 patients) from 183.6 ± 4.58/118.7 ± 2.01 mmHg (mean ± s.e. mean of systolic and diastolic blood pressure) to 171.3 ± 4.08/108.9 ± 2.26 mmHg (P < 0.01) during the administration of atenolol alone. Concomitantly supine heart rate was decreased from 84.0 ± 1.89 to 68.7 ± 1.94 (P < 0.001) beats/min. When atenolol was combined with chlorthalidone, supine blood pressure was reduced from 171.3 ± 4.08/108.9 ± 2.26 mmHg to 143.5 ± 3.68/94.8 ± 2.63 mmHg (P < 0.001). Heart rate did not alter significantly with the addition of chlorthalidone.

4 After the administration of atenolol alone in 12 patients of group B, there was a decrease of mean blood pressure from 131.8 ± 2.88 (mean ± s.e. mean) mmHg to 119.0 ± 4.05 mmHg (P < 0.001); of heart rate from 76.4 ± 3.58 beats/min to 57.0 ± 2.55 beats/min (P < 0.001); of calf blood flow from 9.23 ± 1.39 ml 100 g^-1 min^-1 to 5.05 ± 0.89 ml 100 g^-1 min^-1 (P < 0.001); and an increase of calf vascular resistance from 16.54 ± 1.90 (mmHg min^-1 100 g^-1)/ml to 28.53 ± 3.40 (mmHg min^-1 100 g^-1)/ml (P < 0.005). Atenolol did not alter significantly pre-ejection period index (P < 0.1). In atenolol-treated patients upon addition of chlorthalidone, there was a further decrease of mean blood pressure from 119.0 ± 4.05 mmHg to 105.9 ± 4.12 mmHg (P < 0.001). There were no further significant alterations of heart rate, pre-ejection period index, calf blood flow, and calf vascular resistance (P> 0.01).

5 Atenolol decreased plasma renin activity from 4.69 ± 0.87 to 2.85 ± 0.68 ng ml^-1 h^-1 (P < 0.05), and chlorthalidone increased it from 2.85 ± 0.68 to 3.81 ± 0.98 ng ml^-1 h^-1 (P < 0.05). Plasma renin activity on atenolol plus chlorthalidone was not significantly different from that on placebo (P> 0.1).

6 There was a 7.8 fold-interindividual variability in the relationship between plasma atenolol concentrations and the atenolol dose upon administration of a single oral dose of 100 mg.