1 The effect of metoprolol (200 mg once daily) and guanfacine (2 mg once daily) on 12 h ambulatory blood pressure and heart rate was compared with placebo in a double-blind cross-over randomized protocol. Twenty-six patients with moderate essential hypertension were studied. 2 Metoprolol significantly decreased both systolic and diastolic blood pressure; every point of the blood pressure curve over the 12 h was significantly lower with metoprolol than with placebo and the histograms of systolic and diastolic blood pressure were significantly shifted to the left. 3 Guanfacine significantly reduced systolic and diastolic blood pressure over the 12 h recording period. The antihypertensive effect lasted at least 24 h. 4 There was a significant decrease in heart rate with metoprolol but no significant effect on heart rate could be shown with guanfacine. 5 Neither metoprolol nor guanfacine caused a significant change in the amplitude of blood pressure variations regardless of whether variability was expressed as standard deviation of the mean, as variation coefficient or as range between the highest and the lowest blood pressure.