Many QTLs for biological traits have been reported in animals. However, few QTLs have been characterized in terms of their phenotypic effects taking into account the ontogenic stages of animals used. Body weight in mice has long been used as a model quantitative trait to elucidate the genetic architecture of quantitative variation. Recently, I mapped 24 QTLs for body weight and growth at different ages after birth by genome-wide QTL analyses in an intersubspecific backcross of wild Mus musculus castaneus captured in the Philippines and C57BL/6J, a common laboratory strain. Subsequent analysis using a congenic mouse strain with the wild-derived genomic region harboring one of the QTLs mapped revealed the location of 9 closely linked loci for growth and body composition traits in a small congenic region. These studies revealed that the complex nature of QTL effects (epistatic interactions, sex specificity, age dependence and the issue about linkage vs. pleiotropy) is involved in the regulation of postnatal growth in mice. Here I review how these QTL effects establish a complex QTL network during postnatal growth.