Seventy-seven undergraduates, primed for autonomous or controlled motivation, were videotaped and physiologically monitored during a stressful interview and subsequent speech. Interview videotapes were coded for behavioral measures of threat response; speech videotapes were coded for performance. It was hypothesized that relative to controlled motivation, autonomous motivation would decrease interview threat response and enhance speech performance, and that threat response would mediate the effect of motivation on performance. Results support the prediction across measures of verbal, paralinguistic, smiling, vocal fundamental frequency, and cardiovascular response. Autonomously primed participants continued to show less cardiovascular threat throughout the later speech and gave better speeches. Finally, speech performance was mediated by interview threat response. Results demonstrate that relative to controlled motivation, autonomous motivation lowers threat response, which enhances performance.