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To compare the effects of an angiotensin converting enzyme (ACE) inhibitor and a thiazide diuretic on platelet function and haemorrheological variables, since these factors may contribute to the atherosclerotic and thrombotic complications associated with hypertension.Following a 2-week placebo period, 80 male and female patients with mild to moderate hypertension, aged 50±10 (mean±SD) years, were randomly allocated in a double-blind study to 4 weeks of treatment with the ACE inhibitor lisinopril at 20 mg once a day or the diuretic hydrochlorothiazide at 25 mg once a day. Venous blood was sampled before and at the end of the 4-week treatment period to assess platelet function and haemorrheological variables.Both treatments lowered systolic and diastolic blood pressure equally and had no significant effect on platelet counts and platelet aggregation in response to ADP and to arachidonic acid. Haematocrit plasma viscosity and blood filterability were not altered by either drug. Hydrochlorothiazide tended to increase and lisinopril tended to decrease whole blood viscosity at all shear rates but these changes did not reach statistical significance.Lisinopril increased the erythrocyte aggregation time (from 1.98 ±0.50 to 2.08 ± 0.52 s) and decreased the disaggregation shear rate (from 159 ±46 to 153±40s˜1) and the disaggregation shear stress (from 705 ±257 to 659±204mPa). Hydrochlorothiazide induced the opposite effects (2.00±0.47 to 1.92±0.39s, 181 ±531 to 196±82s~1 and 813 ±268 to 868±392mPa, respectively) with a statistically significant (P < 0.05) intergroup difference.Conclusions: These findings suggest that chronic treatment with the ACE inhibitor lisinopril, but not the diuretic hydrochlorothiazide, may produce favourable effects on blood rheology, but the clinical relevance requires further investigation.