Caspase activation has been implicated in apoptosis, and the nature of the apoptotic stimulus determines the specific caspase activation during apoptosis. In this study, we examined the activation of caspase-3 during photoreceptor degeneration in the rd mouse, which has a mutation on a gene encoding cyclic GMP phosphodies-terase. The outer nuclear layer of therd mouse retina was observed using light and electron microscopy. The progress of degeneration was determined chronologically and correlated with the activation of caspase-3 and the fragmentation of poly-ADP-ribose polymerase. Additionally, the active form of caspase-3 was detected during photoreceptor degeneration in the outer nuclear layer of the rd mouse. The chronological observation of the caspase-3 activation pattern correlates with the pattern of photoreceptor degeneration. As a result of this study, we present here our findings that caspase-3 was activated in photoreceptor cells of the rd mouse. Copyright © 2002 S. Karger AG, Basel.