The antihypertensive effects of clonidine hydrochloride and practolol were compared in 42 men, aged 45 years, who had not received any antihypertensive therapy before, except one patient. The diastolic blood pressures were at least 110 mmHg on two successive visits to the health centre before their selection to the trial. One half of them were classified into the WHO group 1 and the other half into group 2. There was no statistical difference in the systolic and diastolic blood pressures between clonidine and practolol group at the end of placebo period. The study started with a three-week placebo period. Thereafter, 20 patients were given clonidine 0.225 mg and twenty-two practolol 200 mg daily. The next control was carried out after three weeks. The dosage was kept unchanged or increased according to the antihypertensive response. After three weeks, clonidine and practolol dosages were checked again, and 25 mg of chlorothiazide were added to the treatment in 15 clonidine cases and in 18 practolol cases. After the next three-week period, the same regimen was continued on most patients for 6-9 weeks. The daily dosage of clonidine varied from 0.225 to 0.900 (mean 0.394) mg and that of practolol from 200 to 600 (mean 382) mg. Both regimens resulted, when individually adjusted, in a mean systolic blood pressure level of less than 150 mmHg and diastolic pressures less than or equal to 100 mmHg. Hydrochlorothiazide potentiated the blood pressure effect almost equally in both regimens. The blood pressure reduction was statistically significant (p less than 0.05) both in the clonidine and practolol group. There was no significant difference of the mean blood pressures after the active drug therapy between these two groups. A moderate reduction of pulse rate was observed in both main groups, but it was not related to the antihypertensive efficacy. Side-effects were mild. Dryness of the mouth and sedation were more common in patients receiving clonidine. No oculocutaneous or other "immunological" manifestation were seen during the 15-18 weeks' practolol therapy.