This article reviews the available data on the role of the peroxisome in the growth, differentiation and degeneration of mammalian tissues. Developmental progressions of peroxisomes are described, along with the influence of inhibitors of peroxisomal enzymes, peroxisome proliferators and morphogenetic agents on the ontogeny of experimental animals. The role of the peroxisome in protecting tissues from damage by oxygen free radicals is also described, as is the changing role of the peroxisome in the ageing animal. Amongst the degenerative diseases which have been associated with free radical damage are cancer, atherosclerosis, muscular dystrophy, rheumatoid arthritis and the senile degeneration of brain function. In all these conditions, the major characteristics of molecular damage have been considered, along with the particular role of the peroxisome in alleviating these effects. Proposals for further research into peroxisomal function during ontogeny and the degenerative changes associated with ageing are developed, and the possibility of palliative treatments discussed.