In recent years, we have studied cell death in the developing nervous system (Owens et al., 1995), in fetal tissue grafts (Mahalik et al., 1995), and in the rodent olfactory epithelium (Mahalik, 1995). In addition, we are interested in correlating cell death in the rodent CNS with the expression of a putative death-related mRNA called RPS (Owens et al., 1991; Owens et al., 1995; Mahalik et al., 1995). In these studies we have used standard Nissl stains; fluorescent dyes, such as Hoechst 33258 which stain DNA; and the TUNEL method (Gavrieli et al., 19921, in which biotinylated deoxyUTP is incorporated into -hydroxyl groups in nicked or fragmented DNA to identify apoptotic cells in tissue sections. In more recent work, we have combined the TUNEL procedure with immunocytochemistry to identify dying cells in the rodent olfactory epithelium (Mahalik, 1995). Finally, we have used modifications of the TUNEL procedure and other methods to study the aspects of cell death and apoptosis in cultures of PC12 cells and immature rodent thymocytes.