Sixteen men with previously untreated diastolic blood pressure of 100-120 mm Hg were studied hemodynamically at rest supine and sitting and during a 100 W bicycle exercise. Blood pressure (BP) was recorded intra-arterially, and cardiac output (CO) by the dye-dilution method. After initial study, patients were treated with tiapamil in increasing doses (300-600 mg b.i.d., with a mean daily dose of 980 mg). The patients were restudied and then changed to felodipine (5-10 mg b.i.d., with a mean daily dose of 15 mg). Casual (cuff) BP at rest sitting was as follows: end placebo, 166/105 mm Hg; 1 year of tiapamil, 148/92 mm Hg; 1 year of felodipine, 139/86 mm Hg. Pretreatment hemodynamic results at rest sitting were as follows: BP, 169/105 mm Hg; MAP, 129 mm Hg; cardiac index (CI), 2.43 L/min/m2; total peripheral resistance index (TPRI), 4,305 dyn s/cm-5/m2. Both drugs reduced systolic (SAP), diastolic (DAP), and mean (MAP) arterial pressure significantly in all situations. BP reduction seemed to be more pronounced on felodipine than on tiapamil. Felodipine reduced MAP by 15% at rest supine, 14% at rest sitting, and 11% during exercise. The BP reduction was entirely due to reduction in TPRI (15, 16, and 13%, respectively). CI as well as stroke index (SI) and heart rate (HR) were unchanged. Tiapamil did not reduce TPRI significantly and the fall in BP was due to a combination fall in TPRI and CI.