The effect of long-term administration of propranolol (0.2 and 0.4 mg/kg-1 p.o.) and that of labetalol (2.0, 4.0 and 8.0 mg/kg-1 p.o. and 1 mg/kg-1 i.p.) on the development of hypertension was studied in female rats of SHR/N/Ibn strain. The experiment was started on 6-week-old animals. Rats received the tested drugs once daily between 11 and 12 o'clock. From the 29th day on (i.e. after the age of 10 weeks) the arterial blood pressure was measured bloodlessly twice weekly 2 hours before the daily dose of the drug. Labetalol given p.o. in a dose of 2 mg/kg-1 had no effect on the development of hypertension, while the doses 4.0 and 8.0 mg/kg-1 decreased the arterial pressure by about 4 kPa (30 mm Hg). After drug withdrawal a rapid rise was observed in the arterial pressure to values approaching the values recorded in control animals. In the animals receiving labetalol parenterally (1 mg/kg-1 i.p.) the blood pressure was significantly lower than in control rats. After drug withdrawal the blood pressure increased gradually, however, even on the 54th day of the experiment it was slightly below that in controls. Propranolol given orally in doses of 0.2 mg/kg-1 delayed the development of hypertension, while in the dose of 0.4 mg/kg-1 it prevented its development. After withdrawal of the lower drug dose the blood pressure remained for about 2 weeks at a level similar to the previous one, later on it increased steeply to the values observed in controls.(ABSTRACT TRUNCATED AT 250 WORDS)