Fat depot sizes peak in middle age but decrease by advanced old age. This is associated with ectopic fat deposition, decreased adipocyte size, impaired differentiation of preadipocytes into fat cells, decreased adipogenic transcription factor expression, and increased fat tissue inflammatory cytokine generation. To define the mechanisms contributing to impaired adipogenesis with aging, we examined the release of TNF, which inhibits adipogenesis, and the expression of CHOP, which blocks activity of adipogenic C/EBP family members, in preadipocytes cultured from young, middle-aged, and old rats. Medium conditioned by fat tissue as well as preadipocytes from old rats impeded lipid accumulation by preadipocytes from young animals. More TNF was released by preadipocytes from old than young rats. Differences in TNF converting enzyme, TNF degradation, or presence of macrophages in cultures were not responsible. TNF induced rat preadipocyte CHOP expression. CHOP protein was higher in undifferentiated preadipocytes from old than younger animals. Overexpressing CHOP in young rat preadipocytes inhibited lipid accumulation. TNF siRNA reduced CHOP and partially restored lipid accumulation in old rat preadipocytes. CHOP normally increases during late differentiation, potentially modulating the process. This late increase in CHOP was not affected substantially by aging: CHOP was similar in differentiating preadipocytes and fat tissue from old and young animals. Hypoglycemia, which normally causes an adaptive increase in CHOP, was less effective in inducing CHOP in preadipocytes from old than younger animals. Thus, increased TNF release by undifferentiated preadipocytes with elevated basal CHOP contributes to impaired adipogenesis with aging.