In a randomised, crossover study of patients with moderate to severe essential hypertension, the effects of the calcium entry antagonist nifedipine and the beta-receptor blocking drug acebutolol were studied on their own, and in combination. After 4 weeks of nifedipine tablets 20 mg twice daily (Adalat, Bayer), mean supine blood pressure (BP) fell by 20 mmHg and after 4 weeks of acebutolol 200 mg twice a day (Sectral, May & Baker) by 11 mmHg. When nifedipine and acebutolol were given in combination in the above doses for 4 weeks, there was a significantly greater fall in BP than with either agent alone, supine mean arterial pressure falling by 27 mmHg. The above BPs were measured 2 h after the last dose of tablets. Measurements 12 h after the last dose showed smaller falls in BP, with a significantly greater fall with combination treatment than with acebutolol alone. The fall in BP 12 h after the last dose of the combination was greater than with nifedipine alone but this difference was not statistically significant. This randomised, controlled study showed that nifedipine and acebutolol have a marked additive effect on BP which is sustained for at least 12 h after treatment.