The emergence of the whisker-related patterning of the barrel cortex during the first postnatal week is a frequently assessed feature of rodent cortical development and has been used extensively to screen for effects of genetic mutations on neural development in mice. As alterations in body weight often accompany genetic mutations, we asked whether body weight itself might affect the progression of barrel cortex development in wildtype C57/BL6 mice. The body weight varied considerably between as well as within litters, and could differ by a factor of up to 1.6 between littermates. The establishment of the periphery-related and barrel patterning was assessed at postnatal (P days) 4 and 6 using cytochrome oxidase and Nissl staining. We found that only 20% of the mouse pups had an established thalamocortical afferent pattern in the barrel cortex at P4 (4 out of 21 brains), while the majority of the pups showed a well-established pattern at P6 (13 of 16 brains). At both ages the more developed barrel structure was found in the heavier littermates. Conversely, no periphery-related pattern was apparent in the somatosensory cortex at either P4 or P6 below a critical body weight of 2.6 g and 2.0 g, respectively. These findings may have implications for the interpretation of developmental changes in the barrel cortex of genetically modified mice. © 2008 Wiley-Liss, Inc.