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Oxytocin has been implicated in the regulation of prostate growth. However, the cellular localisation of oxytocin in the normal and diseased human prostate is not known. Oxytocin, oxytocin-associated neurophysin and oxytocin receptor were detected by immunohistochemistry in tissues from patients undergoing routine prostatectomy and in normal human prostate epithelial and stromal cell lines. Western blot analysis detected a single band at 14 kDa with neurophysin antiserum and a 66-kDa band with oxytocin receptor antiserum in epithelial and stromal cell lines. Similar sized bands were also detected in extracts of hyperplastic and adenocarcinomic prostate tissues. Oxytocin, oxytocin-associated neurophysin and oxytocin receptor were present in stromal and epithelial cell lines and in tissue from patients with benign prostatic hyperplasia. The peptides were localised predominantly to the epithelial cells, although discrete areas of stromal staining were also observed. There was a significant difference in the intensity of oxytocin-staining between tissue displaying benign prostatic hyperplasia and invasive carcinoma, with less immunoreactivity being present in the malignant epithelial cells. Thus, oxytocin and its neurophysin and receptor are present in epithelial and stromal cells of the human prostate. Oxytocin expression is reduced with tumour progression and may provide a marker for invasive disease.