A differential library screening procedure was used to clone a novel abundant and tissue-specific cDNA from the dog epididymis. It was tentatively named CE7 for dog epididymal gene product 7. By sequence similarity to homologous counterparts expressed in mice, rats, pigs, and macaque monkeys, it appears that the 1.5 kb dog epididymal mRNA encodes the secretory glutathione peroxidase-like protein, GPX5. This protein is very similar to the family of glutathione peroxidase enzymes, but does not contain selenocysteine. Northern blot and in situ hybridization analyses revealed that the mRNA encoding CE7/GPX5, like its species homologues, was restricted to the epididymis and transcribed by the epithelial cells in the proximal parts of the organ. While the CE7 cDNA probe cross-hybridized to epididymal mRNAs in most species included in this study, it failed to identify a human GPX5 counterpart. Northern blot analyses of epididymal RNA extracts from hemi-cryptorchid dogs suggested that testicular secretions, including androgen hormones, temperature effects, or both, were involved in the region-dependent modulation of mRNA encoding CE7 in the dog epididymis. The effect was most obvious in the caput region of the abdominal organ where the mRNA encoding CE7 was almost completely downregulated.