We have previously characterized the evolution of renal cortical interstitial fibrosis in the rabbit model of unilateral ureteral obstruction (UUO). In our earlier report, we examined the extracellular matrix protein composition of the interstitial space. Of note, UUO was associated with the acquisition of prominent interstitial collagen IV immunoreactivity. Interstitial collagens I and III were also increased. In situ hybridization localized increased expression of collagens I and IV to cells of the interstitial space. In the current study, we examine metalloproteinase and metalloproteinase inhibitor expression in the obstructed renal cortex. Matrix metalloproteinase-2 is a metalloproteinase with activity against both collagen IV and denatured collagen I. At day 3 of UUO, both transcripts were significantly increased, although expression of these mRNAs was not different from controls after 7 and 16 days of UUO. Expression of mRNA of tissue inhibitor of the metalloproteinases (TIMP) was significantly increased in the UUO samples at all times, although it was maximal at day 3. Immunohistochemically, increased TIMP reactivity localized to the interstitial space, and TIMP mRNA expression was seen to parallel the interstitial macrophage infiltration that accompanies ureteteral obstruction. In contrast, TIMP-2 mRNA expression appeared to be biphasic, with peaks at both day 3 and day 16 of UUO. At day 7, expression was not different from controls. These data suggest a role for impaired matrix degradation in the development of interstitial fibrosis in the obstructed kidney, particularly at late times when collagen mRNA expression has returned to control values.