Preadipocyte differentiation capacity declines between middle and old age. Expression of the adipogenic transcription factors, CCAAT/enhancer binding protein (C/EBP) and peroxisome proliferator activated receptor (PPAR), is lower in differentiating preadipocytes from old than young animals, while no age-related changes occur in C/EBP mRNA, which is upstream of C/EBP and PPAR. C/EBP-liver-enriched inhibitory peptide (C/EBP-LIP), a truncated C/EBP isoform that is a dominant inhibitor of differentiation, increases with aging in rat fat tissue and preadipocytes. CUG triplet repeat binding protein-1 (CUGBP1) binds to C/EBP mRNA, increasing C/EBP-LIP translation. Abundance and nucleotide binding activity of CUGBP1 increased with aging in preadipocytes. CUGBP1 overexpression in preadipocytes from young animals increased C/EBP-LIP and impaired adipogenesis. Decreasing CUGBP1 in preadipocytes from old rats by RNA interference reduced C/EBP-LIP abundance and promoted adipogenesis. TNF, levels of which are elevated in fat tissue with aging, increased CUGBP1 protein, CUGBP1 binding activity, and C/EBP-LIP in preadipocytes from young rats. Thus, CUGBP1 contributes to regulation of adipogenesis in primary preadipocytes and is responsive to TNF. With aging, preadipocyte CUGBP1 abundance and activity increases, resulting in enhanced translation of the C/EBP-LIP isoform, thereby blocking effects of adipogenic transcription factors, predisposing preadipocytes from old animals to resist adipogenesis. Altered translational processing, possibly related to changes in cytokine milieu and activation of stress responses, may contribute to changes in progenitor differentiation and tissue function with aging.