During milk fat depression (MFD) induced by the feeding of trans-10,cis-12 CLA the magnitude in the reduction of milk fat content and/or yield varies considerably among animals. The objective this study was to evaluate the gene expression of transcription factors and genes downregulated during MFD in lactating ewes fed a uniform dose of trans-10,cis-12 CLA but differing in body weight. Fourteen lactating ewes were selected according to BW and separated in 2 groups that averaged 51.0 ± 1.0 and 69.0 ± 4.0 kg of BW. Ewes received for 10 d a daily oral dose of 27g/d of CLA (rumen-unprotected, 29.9% trans-10,cis-12 isomer). Mammary and adipose tissue biopsies were performed on d 10. RNA was extracted, cDNA synthesized and qt-RTPCR carried out. Data were analyzed using the MIXED procedure of SAS (2002) and ribosomal protein S18 housekeeping gene was used as a covariate. Gene expression of SREBP1 (sterol regulatory element binding protein), S14 (thyroid hormone responsive) and SCD1 (stearoyl-CoA desaturase) were evaluated in mammary tissue samples and leptin in adipose tissue. Gene expression of SREBP1, S14 and SCD1 did not differ between the BW groups (P = 0.29, P = 0.17 and P = 0.57, respectively). Leptin gene expression was higher in the heavier BW group (P = 0.02). Overall, our results suggest that animals of different body weights receiving a uniform dose of trans-10,cis-12 CLA showed change in gene expression of leptin.