The onset of hydronephrosis following unilateral ureteral obstruction is associated with the induced expression of RNA and proteins encoded by the SGP-2 gene. SGP-2 expression has been shown to demarcate mammalian cells undergoing apoptosis. Using in situ hybridization, the cellular localization of SGP-2 expression in the obstructed kidney was determined as a means to study the various phases involved in the progression of hydronephrosis. Within 30 minutes of obstruction, SGP-2 mRNA expression was localized to the adventitial layers of the hilar arteries and intrarenal arterioles. Increasing time of obstruction resulted in the notable absence or depletion of this layer. In addition, the pattern of SGP-2 expression changed with time to the collecting ducts and distal tubules. This study identifies the vascular support tissue of the kidney as the initial site of reaction and potential cell death following ureteral obstruction. We believe that this observation may be of importance in explaining the early alterations in blood flow associated with hydronephrosis.