States characterised by elevated plasma fatty acid levels are accompanied by increased UCP2 expression but the physiological regulation of UCP2 expression in white adipose tissue is not fully understood. We used 3T3-L1 preadipocytes to determine whether various dietary fatty acids (20:5, 18:3, 18:2, 18:1, 18:0) directly regulate UCP2 expression. Physiological concentrations of each class of polyunsaturated fatty acid and the monounsaturated fatty acid dramatically upregulated UCP2 mRNA levels 5- to 8-fold, but the saturated fatty acid was not so effective (1.5-fold). The up-regulation occurred in a time- and dose-dependent manner, was evident by 4 h and maximum between 18 and 24 h, and was prevented by actinomycin D. Synthetic ligands selective for each PPAR isoform did not induce UCP2 expression, which suggests that fatty acids might not be acting solely via PPAR transcription factors. In conclusion, dietary unsaturated fatty acids may be physiological signals to alter energy balance by direct induction of UCP2.