Apoptosis is a form of cell death that plays an important role during physiological homeostasis and numerous pathological conditions. Retinitis pigmentosa is a group of retinal degenerative disorders in which rod photoreceptors excessively die via apoptosis, Current analytical tools for the investigation of photoreceptor cell death are histological in nature and typically time-consuming and laborious; as such, they are frequently Limited to small sample sizes. In recent years, apoptosis research has made extensive use of flow cytometry to analyze several cellular events characteristic of apoptosis, including DNA strand nicking and cell shrinkage. This study presents a how cytometric assay for the detection of rod photoreceptor apoptosis using the rod-specific cell marker rhodopsin, the terminal deoxyuridine 5'-triphosphate (dUTP) transferase nick end-labeling (TUNEL) assay, and the light-scattering properties of rod photoreceptors, This technique enables the rapid and reproducible detection of rod photoreceptor apoptosis in vitro with potential applications for in vivo analysis. (C) 1998 Wiley-Liss, Inc.