Brown adipose tissue is essential in the newborn to protect against hypothermia through thermogenesis and has a crucial role in energy balance in later life in some animal models. Epicardial and pericardial fat depots have both been shown to possess uncoupling protein 1 (UCP1) and function as brown fat [1,2]. The development and modification of these depots in early life in comparison to classical white adipose tissue has not previously been studied. This study focused on the comparison of epicardial, pericardial and omental fat during early postnatal life through gene expression analysis. UCP1 was analysed as a marker of brown or beige fat capable of thermogenesis. Expression of the white fat markers leptin and adiponectin were investigated to elucidate white adipocyte characteristics of each depot. Peroxisome proliferator-activated receptor (PPAR) gamma is involved in white and brown fat differentiation and was assessed as an indicator of mature adipocyte development. Methods: Four triplet-bearing mothers were entered into the study and a randomly selected triplet was euthanased at 1, 7 or 28 days of age for adipose tissue sampling (n=4 per time point) under Home Office Approval, UK. Pericardial adipose tissue was sampled at 1, 7 and 28 days whereas epicardial and omental adipose tissues were only sampled at 7 and 28 days due to a lack of any observable fat in these depots in 1 day old sheep. RNA was extracted and gene expression analysed by qRT-PCR. Relative mRNA expression was calculated using the GeNorm method corrected to the geometric mean of reference genes IPO8 and RPO, and expressed in arbitrary units [3]. Data are presented as mean ± SEM and analysed using one way ANOVA or unpaired t-Test for comparisons with age. Results: With increasing age, UCP1 expression decreased and the white fat marker leptin increased in all depots studied (see Figure 1). UCP1 was detectable in epicardial and pericardial adipose tissue and, to a lesser extent in omental fat, at 7, but not 28, days of age. Adiponectin also increased up to 28 days which was most pronounced in the pericardial fat depot. PPAR gamma increased significantly in all depots with age. Conclusion: Our study suggests a brown to white fat transformation in all adipose tissues during the first month of postnatal life, even in those depots which are undetectable immediately after birth. This may be accompanied by a loss of brown adipose tissue and increased adipocyte differentiation.