Chronic unilateral ureteral obstruction (UUO) results in interstitial fibrosis of the affected kidney. In this study we determined that enalapril ameliorates the increased production of extracellular matrix (ECM) protein in the tubulointerstitium during UUO. The relative volume (Vv) of the tubulointerstitium measured by a point-counting method increased significantly at three or five days of UUO as compared to the contralateral kidney. Enalapril significantly blunted this increase at either three or five days. Immunofluorescence studies revealed that collagen type IV increased remarkably in both the tubular basement membrane (TBM) and the interstitial space at three or five days of UUO. Glomeruli did not show any change. Collagen types I and III were faintly stained in the control kidneys while they were obviously increased in the interstitial space of the obstructed kidney. We examined the expression of collagen type IV (COL IV) because this basement membrane matrix protein appeared to be a major ECM protein deposited in the tubulointerstitium of the obstructed kidney. Semiquantitative analysis of COL IV by immunofluorescence microscopy revealed that enalapril reduced slightly (21%) but significantly (P < 0.01) the deposition of COL IV in the obstructed kidney. Measurement of cyanogen bromide peptides from the obstructed kidney by Western blotting showed an increase of COL IV. This increase was reduced slightly (20%) by enalapril. The level of COL IV mRNA measured by reverse transcription-PCR was very low or undetectable in the control and contralateral kidneys, while it was significantly increased in the obstructed kidney at three or five days of UUO. COL IV mRNA was abundant in glomeruli while it was almost undetectable in renal tubules in the control and contralateral kidneys. However, COL IV mRNA was increased in renal tubules but not in the glomeruli of the obstructed kidney. Enalapril treatment resulted in a 42% decrease (P < 0.01) in COL IV mRNA in the cortex and a remarkable decrease in the renal tubules of the obstructed kidney at five days. Enalapril treatment resulted in an 89% decrease in the number of infiltrating ED-1 positive monocytes/macrophages. These results indicate that enalapril treatment ameliorates the tubulointerstitial fibrosis of the affected kidney in UUO. This effect of enalapril on fibrosis may be due to the severe reduction in monocytes/macrophages capable of secreting the profibrotic factor TGF-β1.