This study was designed to examine changes of hemorheological parameters in essential arterial hypertension subjects following antihypertensive drug therapy. Eighty two female subjects were enrolled, and sub-divided into two groups based upon their high shear whole blood viscosity being lower (L) or higher (H) than normal controls. Equal numbers of L and H subjects were then treated for four weeks with one of four agents: angiotensin-converting enzyme inhibitor (ACE-inhibitor, Spirapril - 6 mg/day); calcium antagonist (Isradipin - 5 mg/day); beta-1-blocker (Talinolol - 100 mg/day); diuretic (Indapamide - 1.5 mg/day). Both prior to and following drug treatment for six weeks, hemorheological measurements included plasma viscosity; high and low shear whole blood viscosity, hematocrit, fibrinogen and RBC aggregation. Treatment with each of the four drugs significantly (p<0.05) reduced blood pressure in both the L and H groups. However, the hemorheological effects of antihypertensive drug therapy differed markedly between groups: plasma and whole blood viscosity were significantly elevated in the L groups whereas these parameters were significantly decreased in the H groups. Fibrinogen levels and RBC aggregation decreased in both groups, whereas hematocrit was unaffected. These results thus suggest that the rheologic effects of antihypertensive drug therapy depend strongly on the initial, pre-treatment status of the subject, and that for some subjects, such therapy can result in adverse hemorheological alterations.