The antihypertensive effects, as assessed by clinical and ambulatory blood pressure measurement, of nifedipine slow-release (SR), atenolol and the two in combination were evaluated in 28 known hypertensives in a placebo-controlled, double-blind, randomised cross-over trial. Clinical blood pressure was significantly lower on combination therapy (P less than 0.025) than on either agent alone, although all therapeutic agents reduced blood pressure significantly when compared with placebo (P less than 0.01). All ambulatory blood pressure measurements obtained on any therapeutic agent were significantly lower than those obtained on placebo (P less than 0.01). The mean daytime (08h00-17h00) ambulatory blood pressure measurement as well as the percentage of this monitoring period during which patients were hypertensive were significantly lower (P less than 0.01) on combination therapy than on nifedipine SR. A similar pattern was observed for 24-hour ambulatory blood pressure measurements. Headache was the most significant adverse effect. This was most common with nifedipine SR, less common with combination therapy and least common with atenolol. Combination therapy with nifedipine SR and atenolol is therefore a viable therapeutic alternative in the treatment of patients with benign essential hypertension.