The role of type IV collagenases during rat bladder development and in response to partial bladder outlet obstruction was evaluated. Gelatinase gel zymography was performed on developing rat bladders (gestation d 16 and 19, at birth, 5, 10, 15, 20, 30, and 75 d postnatally), after partial obstruction of the bladder outlet in young adults and after separation of the epithelium from the mesenchyme in young adults. Bladder function was assessed by cystometry in obstructed animals. During development, the 72-kD type-IV collagenase [matrix metalloproteinase (MMP)-2, both latent and activated] was maximally expressed in the fetal period and decreased with age; whereas the 92-kD gelatinase(MMP-9) was not expressed in developing or adult bladders. MMP-2 was localized to the bladder mesenchyme and was undetectable in isolated epithelium. In 46 obstructed rats, there was an 8-fold increase in bladder volume and weight along with smooth muscle hypertrophy (mean smooth muscle cell diameter 7.09± 0.11 μmversus4.65 ± 0.05 μm in normal animals,p< 0.001). Obstructed rats had increased quantities of latent and activated MMP-2 and MMP-9 compared with sham-operated and normal controls. These findings suggest that expression and activation of type IV collagenases (MMP-2 and 9) are developmentally regulated and play a role in bladder remodeling during developmental morphogenesis and after partial outlet obstruction.Abbreviations: ECM,extracellular matrix;MMP,matrix metalloproteinases