Although apoptosis has been shown in vivo to be involved in the aging process, in vitro studies of age-dependent apoptosis are limited. In this study, apoptosis was examined in normal human fibroblasts exposed to H2O2 to induce premature senescence and in spontaneously senescent human fibroblasts. Although apoptosis was not observed for several days after exposure to H2O2, morpholological changes indicating apoptosis were evident in about 5% of cells 7 days after exposure to 80 muM H2O2,, concomitantly with 14 expression of senescent phenotype. The apoptotic changes were preceded by caspase activation in majority of the exposed cells. As well as H2O2-induced senescent cells, spontaneously senescent human fibroblasts showed apoptotic changes in about 2% of cells and majority of the senescent cells also showed activation of caspases. The results indicate that the apoptosis pathway is activated in H2O2-induced and spontaneously senescent human fibroblasts in vitro. (C) 2004 Elsevier Inc. All rights reserved.