The renal protective effect of antihypertensive drugs is linked to 2 mechanisms. First, reduction in blood pressure (BP) is a fundamental prerequisite common to all antihypertensive drugs. The exact definition of the level to which BP should be reduced remains to be established, although there is some evidence that BP should be reduced below 130/85 mm Hg in patients with diabetic and nondiabetic nephropathies and below 125/75 mm Hg in patients with nondiabetic nephropathies and proteinuria >1 g/day. However, available data suggest that tight BP control (BP<140/80 mm Hg) can reduce the risk of cardiovascular complications in hypertensive patients with type 2 diabetes mellitus (non-insulin-dependent diabetes mellitus; NIDDM). Secondly, intrarenal actions on mechanisms such as glomerular hypertension and hypertrophy, proteinuria, mesangial cell proliferation, mesangial matrix production and probably endothelial dysfunction, which can cause and/or worsen renal failure, are relevant for the renal protective action of some drug classes. ACE inhibitors possess such properties and also seem to lower proteinuria more than other antihypertensive drugs, despite a similar BP lowering effect. Calcium antagonists likewise exert beneficial intrarenal effects, but with some differences among subclasses. It remains to be evaluated whether angiotensin II-receptor antagonists can exert intrarenal effects and antiproteinuric actions similar to those of ACE inhibitors. While primary prevention of diabetic nephropathy is still an unsolved problem. there is convincing evidence that in patients with type 1 (insulin-dependent diabetes mellitus; IDDM) or 2 diabetes mellitus and incipient nephropathy ACE inhibitors reduce urinary albumin excretion and slow the progression to overt nephropathy. Similar effects have been reported with some long-acting dihydropyridine calcium antagonists, although less consistently than with ACE inhibitors. In patients with diabetic overt nephropathy, ACE inhibitors and nondihydropyridine calcium antagonists are particularly effective in reducing proteinuria and both drugs can slow the decline in glomerular filtration rate more successfully than other antihypertensive treatment. Available data in patients with nondiabetic nephropathies indicate that ACE inhibitors can be beneficial, principally in patients with significant proteinuria, in slowing the progression of renal failure. However, it is still unclear whether this beneficial effect of ACE inhibitors is particularly evident in patients with mild and/or more advanced renal failure and whether calcium antagonists possess a similar nephroprotective effect. Overall, data from clinical trials thus seem to indicate that ACE inhibitors and possibly calcium antagonists should be preferred in the treatment of patients with diabetic and nondiabetic nephropathies. However, further information is needed to understand renal protection.