In the ventral prostate of the intact rat, clusterin mRNA is expressed in a small population of cuboidal epithelial cells undergoing apoptosis, but not in the columnar cells comprising the vast majority of the glandular epithelium. Upon castration, clusterin mRNA expression and apoptotic activity are turned off in the cuboidal cells and turned on in the columnar ones. We show here that the progressive enhancements in the abundance of clusterin mRNA, occurring in the rat ventral prostate upon aging, are based on increases of the cuboidal cells at the expense of the columnar ones. DNA fragmentation, a typical sign of apoptosis, assayed both by agarose gel electrophoresis and in situ staining, was undetectable in 3-month-old rats but was evident among the cuboidal cells of 24-month-old animals and columnar cells of 1-day castrates. The DNA content of the ventral prostate did not change significatively between young and old rats, indicating that no increase in the rate of cell death occurs within the age interval examined. It is concluded that the enhancement in cuboidal cell population, the consequent augmented accumulation of clusterin mRNA, and the increased frequency of DNA fragmentation that we have detected in the aging rat ventral prostate are not directly related to the rate of apoptosis.