Most of the publications dealing with the experimental induction of prostatic neoplasia have focused on the description of epithelial lesions but little attention has been paid to the involvement of their stromal alterations. The present study is a first attempt to assess the stromal changes in both collagen and elastic fibrils as well as in its cellular constituents, which accompany prostatic intra epithelial neoplastic (PIN)- like lesions induced by phenylephrine (PE) in rats. Adolescent rats received subcutaneous injections of PE daily (10mg/kg/ day) for one month. At the end of the experimental period the rats were euthanized, the dissected ventral prostates were fixed in Stieve solution, paraffin embedded, sections were cut and stained accordingly. Most of the stromal cells were identified by Immuno-histochemistry (IHC) techniques using primary antibodies to ED2 (resident macrophages), actin (fibrocytes and vascular smooth muscle cells), vimentin (mesenchymal cells) and BrdU (S-phase proliferating cells). Collagen stromal mass was visualized by Gomori trichrome and individual collagen fibers by picro- sirius red staining under polarized light, whereas the fine fibrils were stained according to the Pinkus method. The untreated rat prostates are characterized by a delicate interacinar stroma with scanty cells and fibrils. The PE treated prostates showed a significant increase in both cellular and fibrilar elements as well as an increase in arteriolar density besides the typical PIN lesions. The presence of such an interstitial fibrosis which also includes inflammatory cells, neoangiogenesis and synthesis de novo of collagen and fibers, might be regarded as a desmoplastic reaction. It is suggested that these changes could be related to a tissue repair process subsequent to the inflammatory exudate which takes place during the incipient phases of the PE treatment.