The rat prostate is a complex ductal system with branches and subbranches extending from one end to another. Owing to the relative distance of various regions from the urethra, the entire length of the ductal system can be divided into three segments, i.e. the proximal, intermediate, and distal segments. The present study was carried out to examine the pattern of localization of sulfated glycoprotein-2 (SGP-2), a marker protein associated with programmed cell death, in various regions of the prostatic ductal system under normal conditions and during castration-induced regression. SGP-2 has been considered an androgen-repressed gene product in the rat prostate and has previously been known as castration-induced protein or TRPM-2. In the normal rat prostate, immunoreactive SGP-2 was localized in epithelial cells lining the proximal segment in which signs of programmed cell death were apparent. Cells lining the distal and intermediate segments were, however, devoid of SGP-2. This observed regional variation in SGP-2 localization did not support an earlier hypothesis which stated that SGP-2 was constitutively expressed by all prostatic epithelial cells in the normal rat prostate. After castration in adult rats, there was a shift in the location of cells containing SGP-2 from the proximal segment toward the distal segment. Therefore, there is a regional variation in the distribution of SGP-2 in the rat prostate both before and after castration in the host. These findings are likely to be associated with a regional variation in cellular responsiveness to androgen stimulation and androgen depletion in the prostatic ductal system. Results also support the view that SGP-2 localization is associated with an early manifestation of programmed cell death in the rat prostate. (Endocrinology128: 2091–2102,1991)