Continuous administration of the antioxidant nordihydroguairetic acid to clones of the natural-death nutant or wild-type Neurospora crassa growing on 5-fluorotryptophan not only alleviates the time-dependent deterioration of growth rate (senescence), but also inhibits the accumulation of a fluorescent pigment (lipofuscin), an end-product of lipid peroxidation. In the three model system of ageing in N. crassa that we have examined, it is now clear that massive accumulation of lipofuscin begins shortly before or concident with the period of senescence. By this and other criteria, the youngest cells in time of origin appear to be most senescent. Thus the degree of senscence appears to be a function of physiological rather than chronological age and, more specifically, is probably a function of the number of nuclear mitotic divisions. Normal, non-senescent, wild-type, clones also accumulate lipofuscin; but, unlike clones undergoing senscence, accumulate very low concentrations which are independent of subclonal age. Apparently, clones of wild type can tolerate a low level of lipid peroxidation without undergoing senescence; but even a low level is inimical since culture with nordihydroguaiaretic acid not only enhances their growth rate, but also inhibits the accumulation of lipofuscin.