Fetal and adult heart muscles differ markedly at molecular, structural and physiological levels. There is a great interest in identifying those differences, their underlying mechanisms and downstream effects, particularly in light of reports showing that in certain pathologic conditions, leading to heart failure, the adult heart reverts towards a “fetal” gene expression program(1, 2). In the era of high throughput technologies it is possible to undertake wide-ranging cardiac protein expression profiling. Therefore, using LC-MS-based approaches, we have performed a proteome scale comparison of fetal and adult guinea pig heart tissue (left ventricular), to identify proteins and pathways involved in heart development/maturation. Adult female (pregnant and non-pregnant) guinea pigs were killed according to Home Office guidelines and excised fetal (gestation day 65-67, term ~day 67) and adult hearts flushed clear of blood (glucose 227.5 mM, mannitol 34.5 mM, KCl 30 mM, NaHCO3 25 mM, pH 7.4) and stored frozen for subsequent analysis. Two separate protein extraction methods were applied in an attempt to combat issues of a massive protein dynamic range typical for muscle tissues (100mM Tris base pH7.6, 4% SDS, 0.1M DTT, 20μl/ml protease inhibitor (Sigma P8340) for total lysate and 4mM EDTA, 2mM EGTA, 5mM DTT, 150mM sucrose, 20μl/ml protease inhibitor for myofibrils depletion). Lysates were trypsinized and analyzed using two separate instrument platforms (Thermo Q-Exactive Plus in Data Dependent Acquisition mode and AB-Sciex TripleTOF-6600 with SWATH acquisition). The raw data was searched against a customized guinea pig proteome database and quantified using MaxQuant (Thermo) or Peak View (AB-Sciex). Statistical analysis of a combined dataset was performed in R. Under our confidence criteria (2 unique peptides per protein with FDR <0.01, quantifiable for each of 3 biological replicates in at least one experimental condition) we have quantified 2158 unique proteins. Expression of 678 proteins was significantly different (Student t-test, FDR <0.05, fold change >1.5) with 489 more abundant in fetal and 189 in adult heart muscle. Subsequent pathway analysis in String and PathVisio revealed reduced fetal cardiac expression of proteins essential to branched-chain amino acid (BCAA – leucine, isoleucine, valine) degradation e.g. Branched Chain Amino-Acid Transaminase (BCAT2), Branched Chain Keto Acid Dehydrogenase (BCKDHA, BCKDHB), Dihydrolipoamide Branched Chain Transacylase (DBT). This suggests that regulating the expression of proteins key to BCAA catabolism is important for physiological cardiac remodeling and identifies these proteins as interesting candidates to study in the progression of hypertrophic cardiopathologies.