The objective of this study was to assess the antihypertensive effect and the trough to peak (T:P) ratio of lisinopril and captopril, in patients with essential hypertension. After 2 weeks of placebo, 69 of 115 eligible patients had office diastolic blood pressure (DBP) between 90 and 114 mm Hg and daytime average DBP above 85 mm Hg during a 25-h ambulatory BP monitoring (ABPM) and were randomised to receive lisinopril (20 mg once daily) or captopril (50 mg twice daily) for 4 weeks. Office and ambulatory BP were then repeated. Indices of 24-h BP and T:P ratios were calculated and compared. Both drugs significantly reduced both office and ambulatory BP. The final BP obtained with lisinopril was less than with captopril. On office measurement, 75% of the patients treated with lisinopril and 44% on captopril were controlled (P < 0.001), but responses by ABPM were not significantly different. T:P ratios calculated in all patients were 0.75 and 0.66 for lisinopril and captopril respectively, but in patients who responded to each drug the corresponding ratios were 0.78 and 0.73. In conclusion both 20 mg once-daily lisinopril and 50 mg captopril twice-daily achieve a favourable T:P ratio in patients with essential hypertension.