This study was designed to examine changes of hemorheological parameters and red cell aggregation particularly in essential arterial hypertension subjects receiving antihypertensive diuretic therapy. Fifty six female subjects were enrolled in this study. Thirty seven subjects (group I) were treated for four weeks with Hydrochlorothiaszide (25 mg/day); Nineteen patients (group II) were infused with dose of furosemide 40 mg i.v. Both prior to and following drug treatment for four weeks and four hours after furosemide infusion hemorheological measurements included plasma viscosity; hematocrit, total plasma protein, red cell rigidity index (Tk) and RBC aggregation indices. In addition to this protocol the erythrocytes of patients of group II were incubated with furosemide (0.03 mM; for 30 min at 37 degrees C) to study a direct furosemide effect on red cell aggregation. Treatment and infusion with each of the two drugs significantly (p<0.05) reduced blood pressure in both groups. However, the hemorheological effects of hydrochlorothiaszide therapy were not significant. The effect of furosemide infusion and red cell incubation with it led to significant RBCA elevation. These results thus suggest that the rheologic effects of saluretic diuretics therapy were not significant. Single furosemide infusion and using it in vitro resulted in strong effect of red cell aggregation increase.