Some members of the Prx family are up-regulated in cells under stress conditions. Prx I is the major cytoplasmic Prx and is known as a stress-inducible antioxidant enzyme. Various stress agents or conditions activate Prx I gene expression in vitro and in vivo. The transcription factor Nrf2 and its inhibitor Keap1 play an essential role in the regulation of the stress-induced Prx I gene activation through the ARE/EpRE (antioxidant/electrophile response element). The expression levels of Prx II and III are also up-regulated under stress conditions, although the molecular mechanisms of their up-regulation have not yet been thoroughly studied. Gene expression of both Prx I and II is activated by X-ray irradiation of the testis. Mitochondrial Prx III is up-regulated by stress agents in both cultured cells and experimental animals. The up-regulation of the Prxs in cells and tissues under oxidative stress conditions is one of the cellular recovery responses after oxidative damage.