The effects of a single dose regimen of lisinopril (10mg daily) and quinapril (10mg daily) were studied in 47 patients with mild to moderate essential hypertension. After discontinuation of previous antihypertensive treatment and a 2-week washout period, all patients received placebo for the next 2 weeks. Patients were then randomly assigned, in a double-blind fashion, to one of two groups: the first group (group L) was made up of 24 patients who received lisinopril 10mg once daily; the second group (group Q) was made up of 23 patients who received quinapril 10mg once daily. The duration of treatment was 12 weeks. Systolic (SBP) and diastolic blood (DBP) pressure and heart rate (HR) were measured every 2 weeks. Routine blood and urine investigations were performed at weeks 0, 8 and 12. Two weeks after initiation of treatment, a statistically significant reduction in both SBP and DBP was observed in both groups. Comparison between the 2 groups showed a significant reduction in SBP (p < 0.05) at week 4 and in DBP (p < 0.05) at week 8, in favour of the quinapril group (group Q). In group L, heart rate remained unchanged, while in group Q a significant reduction was observed from week 8 until the end of the study (p < 0.001). At week 12, a significant increase in serum potassium was observed in both groups compared with baseline levels (p < 0.01), without significant differences between the 2 groups. The other parameters did not show any significant change during the study. In conclusion, both drugs exhibit adequate antihypertensive effect, but quinapril seems to cause a greater fall in blood pressure and HR 4 weeks after initiation of treatment.