Mast cell and testosterone interactions involved in renal fibrosis in rats subjected to unilateral ureteral obstruction (UUO) were investigated. Orchiectomized (ORX) and nonorchiectomized Wistar rats were subjected to UUO, and a nonorchiectomized group was sham-operated (control: SO). Animals from the UUO group were treated with saline or sodium cromoglycate (CG). Some ORX rats from the saline or CG groups also received testosterone propionate replacement (TR). Kidneys and blood were collected 14 d after UUO or SO. Kidney sections were stained with toluidine blue to quantify mast cells, and picrosirius red was used for collagen analysis. Immunohistochemistry for α-smooth muscle actin (α-SMA) and proliferating cell nuclear antigen (PCNA) expression was also performed. Plasma testosterone levels (PTLs) were measured. ORX decreased and TR normalized PTLs. UUO increased mast cell density in the kidney pelvis, but not in the kidney parenchyma. UUO increased mast cell degranulation, and CG or ORX inhibited this effect. TR partially reversed the effect of ORX on mast cell degranulation, and CG partially inhibited that effect of TR. UUO increased the collagen areas of the renal parenchyma, whereas CG or ORX abolished that alteration; TR reversed the effects of ORX, and CG partially inhibited that effect of TR. UUO increased tubulointerstitial α-SMA expression and PCNA-positive cells, and these changes were sensitive to ORX or CG to the same degree, while TR again reversed the effect of ORX. Renal fibrosis after UUO appears to be determined by interactions between testosterone and mast cells.