In a double-blind cross-over multicentre study lasting for 16 weeks, the antihypertensive effect of atenolol (100 mg once a day) was compared with that of methyldopa (250 mg thrice a day) in 48 patients with mild to moderately severe hypertension. At the end of the trial, atenolol was superior to methyldopa in achieving clinically relevant BP levels, less than 150/95 mmHg (p less than 0.001), and in respect to patients' drug preference (p less than 0.04). The BP levels after the first 2 weeks of treatment with either drug were identical but, in contrast to methyldopa, atenolol caused further significant reduction in BP during the following 4 weeks. This finding implies that one should hesitate to start additional therapy until after at least six weeks of atenolol treatment, whereas no further reduction in BP can be expected after the first 2 weeks of treatment with methyldopa. Two patients had to be withdrawn from the study during treatment with atenolol, as they developed severe bradycardia and dyspnoea, respectively. Other side-effects were few and of minor clinical importance.