The quality of the early life environment has a dramatic impact on stress reactivity and anxiety in adulthood. This dissertation examined the relative contributions of genetic and early environmental factors on behavioral, endocrine, and neurobiological differences in two inbred mouse strains, C57BL/6J and BALB/cJ, that show dramatic differences in maternal care, stress reactivity, and anxiety. Significantly lower levels of maternal care were observed in BALB/cJ compared to C57BL/6J dams, and heightened stress-induced plasma corticosterone levels were measured in BALB/cJ compared to C57BL/6J adult mice. Litters of C57BL/6J and BALB/cJ mice were cross-fostered and anxiety-like behavior, stress reactivity, and neuroanatomy were assessed in adulthood. Cross-fostering produced increased emotionality in the novel open field in C57BL/6J mice reared by a BALB/cJ dam versus a C57BL/6J dam. In addition, cross-fostering attenuated anxiety-like behavior in the elevated plus maze in BALB/cJ mice raised by a C57BL/6J dam as opposed to a BALB/cJ dam. Cross-fostering also elevated basal corticosterone levels in C57BL/6J mice reared by a BALB/cJ dam compared to a C57BL/6J dam. Receptor autoradiography in cross-fostered mice revealed alterations in receptor binding due to maternal care. Specifically, BALB/cJ mice reared by a C57BL/6J dam had increased binding of dopamine D1-like receptors in the nucleus accumbens compared to those reared by a BALB/cJ dam. C57BL/6J mice raised by a BALB/cJ dam, versus a C57BL/6J dam, had decreased levels of the dopamine transporter in the nucleus accumbens and the caudate putamen. However, there were no differences in D2-like receptor binding in the areas tested and no significant findings in CRF receptor binding in the anterior cingulate cortex or the bed nucleus of the stria terminalis in cross-fostered C57BL/6J and BALB/cJ mice. These results suggest that while genetic differences account for some of the variations between these two strains, the early postnatal environment can alter the development of the stress response and anxiety and brain areas that regulate these behaviors.