Female mice are much more resistant to ischemia/reperfusion (I/R)-induced kidney injury when compared with males. Although estrogen administration can partially reduce kidney injury associated with I/R, we demonstrated that the presence of testosterone, more than the absence of estrogen, plays a critical role in gender differences in susceptibility of the kidney to ischemic injury. Testosterone administration to females increases kidney susceptibility to ischemia. Dihydrotestosterone, which can not be aromatized to estrogen, has effects equal to those of testosterone. Castration reduces the I/R-induced kidney injury. In contrast, ovariectomy does not affect kidney injury induced by ischemia in females. Testosterone reduces ischemia-induced activation of nitric oxide synthases (NOSs) and Akt and the ratio of extracellular signal related kinase (ERK) to c-jun N-terminal kinase (JNK) phosphorylation. Pharmacological (Nomega-nitro-L-arginine) or genetic (endothelial NOS or inducible NOS) inhibition of NOSs in females enhances kidney susceptibility to ischemia. Nitric oxide increases Akt phosphorylation and protects Madin-Darby canine kidney epithelial cells from oxidant stress. Antagonists of androgen or estrogen receptors do not affect the gender differences. In conclusion, testosterone inhibits the post-ischemic activation of NOSs and Akt and the ratio of ERK to JNK phosphorylation through non-androgen receptor-medicated mechanisms, leading to increased inflammation and increased functional injury to the kidney. These findings provide a new paradigm for the design of therapies for ischemia/reperfusion injury and may be important to our understanding of the pathophysiology of acute renal failure in pregnancy where plasma androgen levels are elevated.