Although the content of mitochondrial enzymes in different tissues can vary greatly, understanding the regulation behind these differences has been hampered by a lack of quantitative knowledge in relation to postnatal development. Here we report a quantitative analysis of developing brain, heart, kidneys, and muscle tissue of C57BL/6J mice, focusing on the content of mitochondrial complex I, a key component of the respiratory chain: We found that in all tissues except kidneys, complex I content gradually increases after birth, reaching a plateau level at around 25 days. Complex I content in muscles does not change significantly until postnatal day 7-10, and then also increases. The greatest increment was found in kidneys, where a 16-fold increase in complex I level after birth was observed. We also found that content of complex I in all postnatal tissues, but muscle, is higher in males than in females. These baseline dynamics of this key mitochondrial flavoprotein serve as a reference for evaluating genetic influences on development and provide a standard for assessing mitochondrial complex I function during postnatal growth.