Abstract. Equivalent long-term effects on the kidney are attributed to angiotensin-converting enzyme inhibitors (ACEI) and angiotensin II type 1 receptor blockers (ARB). Nevertheless, it is unknown to which degree effects of these compounds on individual inflammatory mediators, including matrix metalloproteinases (MMP), are comparable. On the basis of structural and functional differences, it was hypothesized that ACEI and ARB differentially regulate MMP activity. In a randomized, prospective crossover trial, the effect of an ACEI (fosinopril; 20 mg/d) and of an ARB (irbesartan; 150 mg/d) on MMP activity was evaluated. Ten hypertensive patients with glomerulonephritis and normal or mildly reduced creatinine clearance were studied. MMP activity and tissue inhibitors of metalloproteinase (TIMP) levels were analyzed in serum and urine: without therapy, with ACEI, with ARB, and with both agents combined. Treatment periods continued for 6 wk separated by periods of 4 wk each without therapy. Untreated patients with glomerulonephritis displayed distinctively higher serum levels of MMP-2 but much lower MMP-1/-8/-9 concentrations compared with healthy control subjects. Immunohistology of MMP-2 and MMP-9 in kidney biopsy specimen was accordingly. However, these patients excreted higher amounts of MMP-2 and MMP-9 in urine than healthy control subjects, possibly reflecting ongoing glomerular inflammation. In patients with glomerulonephritis, ACEI significantly reduced overall MMP serum activity to 25%, whereas ARB did not show any effect. Activities of MMP-1/-2/-8/-9 were also significantly inhibited by fosinopril but not by irbesartan. Levels of TIMP-1/-2 remained unaffected. In conclusion, ACEI and ARB differentially regulate MMP activity, which may ultimately have consequences in certain types of MMP-dependent glomerulonephritis.Glomerular inflammatory diseases represent frequent and often difficult-to-treat causes of end-stage kidney failure. Hypertension and proteinuria are important independent determinants and risk factors for progression of these diseases (1). Therefore, these two conditions are prime targets for the therapy of all types of glomerulonephritis.Pharmacologic inhibition of the renin-angiotensin system by angiotensin-converting enzyme inhibitors (ACEI) and angiotensin II type 1 receptor blockers (ARB) has been particularly well demonstrated to treat high BP and proteinuria successfully (1). Moreover, the decline of kidney function occurring in diabetic as well as in nondiabetic glomerulopathies was greatly reduced by these agents in many investigations (2,3). Although ARB have only recently been introduced to common clinical use, several studies reported equivalent renoprotection for both classes of agents, ACEI and ARB (1). However, the issue of long-term effects with regard to the kidney is not yet fully resolved, and the identification of patient groups in which one of these agents demonstrates potential advances over the other one remains of pivotal clinical interest.Angiotensin II plays a key role...