Glucocorticoid levels in animals may respond to and influence the development of social attachments. This hypothesis was tested in prairie voles (Microtus ochrogaster), monogamous rodents that form long-term heterosexual pair bonds. In socially naive female prairie voles, cohabitation with an unfamiliar male resulted in a dramatic decline in serum corticosterone levels. When corticosterone levels were reduced via adrenalectomy, females developed partner preferences after 1 h of cohabitation, while sham-operated and untreated females required 3 h or more of nonsexual cohabitation to establish a partner preference. In adrenalectomized and intact females, exogenous injections of corticosterone, given prior to social exposure, prevented the development of preferences for the cohabitating male. Although corticosterone inhibited the development of partner preferences, it did not interfere with the expression of previously established social preferences. These results suggest that social stimuli can modulate adrenal activity and that adrenal activity, in turn, is capable of influencing the formation of adult social preferences in female prairie voles. The involvement of the adrenal axis in the formation of partner preferences and the subsequent development of pair bonds provides a mechanism through which environmental and social factors may influence social organization in this species.