By interrupting the integrity of the systemic and renal renin-angiotensin system, angiotensin-converting enzyme inhibitors have been shown, experimentally, to preferentially reduce postglomerular capillary arteriolar resistance, to reduce glomerular capillary pressure, and to increase the ultrafiltration coefficient. Under normal physiological conditions, angiotensin-converting enzyme inhibitors have little effect on glomerular filtration rate; however, they increase effective renal plasma flow at renal perfusion pressures within the normal autoregulatory range and renal vascular resistance is decreased. In contrast, calcium antagonists have been shown, experimentally, to preferentially reduce preglomerular capillary arteriolar resistance. Their effects on angiotensin II and postglomerular capillary arteriolar resistance (hence, glomerular capillary pressure and the ultrafiltration coefficient) are controversial. Under normal physiological conditions, calcium antagonists increase both glomerular filtration rate and effective renal plasma flow at renal perfusion pressures within the normal autoregulatory range and renal vascular resistance is decreased. In patients with essential hypertension, studies have demonstrated that angiotensin-converting enzyme inhibitors (as predicted) sustain glomerular filtration rate, increase effective renal plasma flow, and decrease renal vascular resistance. However, essential hypertensive patients with impaired glomerular filtration rate may demonstrate marked improvement in both glomerular filtration rate and effective renal plasma flow. Calcium antagonists (as predicted) may increase both glomerular filtration rate and effective renal plasma flow (at high renal perfusion pressures) and may decrease renal vascular resistance. Calcium antagonists may also improve both glomerular filtration rate and effective renal plasma flow in patients with impaired glomerular filtration rate. Long-term clinical trials comparing the renal effects of angiotensin-converting enzyme inhibitors with those of calcium antagonists in essential hypertensive patients have not been reported. It remains to be determined if the potentially different effects of these two classes of antihypertensive drugs on the renal microcirculation do or do not translate into different renal protective advantages to patients at risk for the development and/or progression of hypertensive nephrosclerosis.