Previous studies have demonstrated that the rapid involution of the rat ventral prostate following castration involves the death of the androgen-dependent epithelial cells present within the gland and that this death is the result of a series of discrete biochemical steps. The degradation of genomic DNA into nucleosomal-sized fragments is an early event in this process and is catalyzed by calcium magnesium-dependent endonuclease activity. The morphologic correlation of the involution process involves a series of structural changes which are collectively referred to as apoptosis. The apoptotic process describes the earliest apparent signs of morphologic change exhibited by the dying cells through their eventual complete destruction and deletion from the tissue. The temporal relationship between these recently described biochemical events and the morphologic changes of the apoptotic process were compared in the present study, in order to test the cause versus effect nature of DNA fragmentation in the programmed death of androgen dependent prostatic cells following castration. These studies demonstrated that the early elevation of the Ca+2 Mg+2-dependent endonuclease activity and the fragmentation of DNA into nucleosomal oligomers occurs within prostatic glandular epithelial cells and probably does not involve the direct participation of extraprostatic cells which may subsequently migrate into the gland. Once the DNA is initially cleaved into the nucleosomal oligomers, the subsequent participation of lysosomal enzymes act in a less restricted fashion to degrade both the nucleosomal DNA as well as the cytoplasmic elements and the cell becomes morphologically apoptotic. As the elevations in Ca+2 Mg+2-dependent endonuclease activity and DNA fragmentation are initiated at a time well before the cell is morphologically dead, as defined by apoptosis, these changes in DNA metabolism must not be the consequences of cell death but instead are early causal events in an active process of programmed cell death.