Co-rumination is a form of interpersonal emotion regulation wherein dyads engage in extensive, cyclical conversations regarding the causes and consequences of problems and associated negative emotions. In the present investigation, we leveraged the biopsychosocial model of challenge and threat to elucidate the intrapersonal costs and interpersonal benefits of co-rumination. To do so, we developed the first direct experimental manipulation of co-rumination using a multimethod, dyadic approach to test the effects of co-rumination on both dyad members. Friend dyads (N = 172) engaged in conversation during which one dyad member (i.e., the discloser) divulged and discussed their most stressful, extradyadic problem with their friend (i.e., the responder). Dyads either engaged in co-rumination or talked about the problem as they would naturally. Validating the experimental paradigm, results revealed a pattern of intrapersonal costs (stressed/upset feelings and rumination) and interpersonal benefits (perceived partner responsiveness) of co-rumination that replicated and extended past research. Regarding challenge and threat, results indicated that female disclosers in the co-rumination (vs. natural) condition exhibited physiological responses corresponding to greater psychological threat (i.e., greater total peripheral resistance). This research contributes to a growing body of literature identifying co-rumination as a vulnerability factor that exacerbates stress, potentially leading to poor downstream health outcomes. Furthermore, these results highlight the importance of examining co-rumination from a dyadic perspective, as inter- and intrapersonal effects varied as a function of whether individuals were disclosing or responding during the problem talk discussion. (PsycInfo Database Record (c) 2023 APA, all rights reserved).