The aim of the present paper was to study 24-h blood pressure profile in twenty patients with essential hypertension (mild to moderate) before and after treatment with ACE inhibitor enalapril in one day dose in the morning (15.6 +/- 6.1 mg). 24-h ambulatory blood pressure monitoring was examined after 14 days of placebo administration and after one month of enalapril therapy. Enalapril treatment decreased systolic blood pressure of 11.3 mmHg during daytime and of 9.4 mmHg at night, diastolic blood pressure of 6.1 mmHg during daytime and 5.8 mmHg at night, all differences were significant in comparison to placebo (p < 0.01). The through-peak ratios were 63.2% for systolic blood pressure and 54.0% for diastolic blood pressure. We can conclude that enalapril treatment in one day morning dose is efficient to decrease blood pressure during 24-h span without altering the diurnal profile.