Previous research has demonstrated that the cardiovascular system (CVS) is not completely developed in the rat until 20 days of age. Prior to this age the sympathetic nervous system (SNS) is dominant during resting conditions and the stress response is vagally mediated, which is the opposite of most adult rats. In the present research, using the Bailey recombinant inbred strains and their progenitor strains (C57BL/6By and BALB/cBy), we determined the developmental period for the CVS in mice. The CVS was controlled by a high SNS tone and the stress response was vagally mediated at 8 days of age. By 12 days of age, the mice had much slower baseline heart rates which were vagally controlled. Significant strain differences were also found with a polygenic model accounting for these differences. The stress response of the 12-day-old mice was not interpretable because of individual differences in the adult coping strategies. These differences were accounted for by a different polygenic model than was found in the baseline condition.