Infusion of renal side population (SP) cells, enriched with adult stein-like cells, can ameliorate acute renal failure. We investigated the effects of an angiotensin II type 1 (AT(1)) receptor antagonist, valsartan on SP cell changes in renal injury by ureteral obstruction. Renal SP fraction was reduced by 38%. and the number of cells expressing CD45, a marker of hematopoietic system, in renal SP cells was increased in obstructed kidneys. Valsartan attenuated renal injury and the associated SP profile changes. Angiotensin AT(1) receptor blockade may exert regenerative effect by preserving adult stem-like cells such as SP cells in the kidney. (c) 2007 Elsevier B.V. All rights reserved.