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The temporal blood pressure course and the diurnal profile of the renin–angiotensin system were examined in 13 patients with essential hypertension receiving hydrochlorothiazide and enalapril once daily. Blood samples were taken and blood pressure was measured before the habitual morning dose of hydrochlorothiazide and enalapril (at 8.00 a.m.) and at seven time points over the next 24 h. During the period of maximal effect of enalapril (from 11.00 a.m. to 2.00 p.m.), the increase in plasma renin concentration ranged from no change to an 800% increase. A negative correlation was observed between an increase in plasma renin and a decrease in immunoreactive plasma angiotensin II concentration (Spearman rank-order correlation coefficient=0.83). Notably, the greatest changes in plasma renin and angiotensin II concentrations after enalapril were seen in those patients whose blood pressure fell most during the day. We conclude that hypertensive patients on long-term therapy with enalapril once daily vary widely in their between-dose biochemical response to the drug, and that there is a significant association between the responsiveness of the plasma renin–angiotensin system and the effect on 24 h blood pressure.