A clinical study is described in which hypertensive patients on no drug therapy were given atenolol in increasing doses from 50 mg twice daily to 200 mg twice daily until the diastolic blood pressure was 90 mm Hg or below. At this stage the drug was withdrawn until blood pressure readings had risen to pre-treatment levels. The same dose of atenolol was then re-introduced but now given once-a-day (in the morning) and was continued for four weeks. Of the elevel patients entering the study, one withdrew as his blood pressure was not controlled on a dose of 200 mg of atenolol twice daily, and another because on withdrawal of atenolol his blood pressure did not rise to pre-treatment levels. The remaining nine patients completed the study. There was a statistically significant fall in blood pressure on both atenolol regimes and there was no significant difference between the blood pressure control achieved on twice-a-day and once-a-day administration. Only one patient developed side-effects; this being an asthmatic who developed mild dyspnoea on atenolol which did not necessitate withdrawal of the drug. It is concluded that once-a-day administration of a given dose of atenolol is therapeutically equivalent to twice-a-day administration. The implications of this in terms of better patient compliance, and thus better prognosis, are discussed.